If this is not a pattern for your step-son, then I think 4 weeks with no friends, computer and phone is harsh. Removing any one of those seems more reasonable to me.
You say he's a good kid but that he has "difficulty following orders like instructed." Does he have any input into his life at your house? At 16 he should be having some responsibility in making decisions so that he can learn how to make good decisions.
Did he know that 15 was your choice for the age to be able to babysit? If you haven't talked about the age of 15 for babysitting then he may have not thought of it. Did the kids left behind know where he went? If they told you then I understand the lack of a note or phone call.
I think he probably make a good choice for the welfare of the children. but since you told him to babysit until you returned he made a poor choice to leave. Did you sense that he was trying to pull one over on you? Could this incident just be one of a thoughtless choice?
Yes, he should be disciplined but I think 4 weekends has the potential to discourage him from doing the right thing because he will think it's unfair and be angry on the inside even if he doesn't show it on the outside.
It sounds like his father and you do not co-operate with discipline decisions? I think a more reasonable consequence would be to "ground" him from his friends and phone for a week, including the week he's with his father and the weekend he's with you. Is it possible that a part of your reason to make the discipline to be 4 weekends is related to how his father doesn't co-operate and might have been chosen in part because you were angry at him and his father.
Please note that I use the word discipline. The purpose of discipline is to teach the child the importance of self-discipline. Punishment, to me, means something done to the child. Punishment is meted out by the authority without discussion with the child about his behavior. I think punishment lets the child off the hook. He serves his time and is free to do what he wants as long as he accepts his punishment. Discipline is teaching a lesson so that the child knows how to act better after the discipline is completed.
If leaving the children is not a pattern and he were my son, I would talk with him about the reasons that doing so was inappropriate. Tell him you agree that the younger children were OK but that he should have asked your permission before leaving. Ask him what he was thinking at the time he made the decision to leave. Talk with him. Not at him. And not in anger. When I've been angry I tell my grandchildren that I will talk with them about the consequences when I've cooled down. This is effective not only because I've had time to think it thru but it also causes them to have more time to be worried about the consequences which is in itself a consequence.
I picked up on the phrase following orders. I believe it's important to respect the child as having the ability to respond to my requests without giving orders. Giving orders is a last resort when the child is not willing to co-operate. Most kids resent orders because it takes away their independence and ability to participate in making decisions affecting them. So I would ask your son to watch the kids. I would praise him every time he does what you ask. I would enlist his co-operation instead of ordering him to do something.
When my daughter is tired and/or upset about something she will order her kids to do things. They respond slowly if at all. When she asks them to do something they usually quickly do it. Not always. Kids have to test boundaries to learn how to fit in their world.
If you think your son was genuinely sorry then he may not need an additional consequence. He tested the boundary, learned that he didn't like the way it turned out and was sorry. Since he has difficulty following "orders" (hopefully you don't have the same definition of orders as I do) I would give him a consequence. The first time I "ground" a kid, the consequence is light. If the misbehavior continues the consequences get more serious. Always I have a conversation, stating my feelings and concerns and allowing the kid to do the same. It's a lesson I'm teaching. Since this is the first time he's left the kids without permission I might not give a consequence if I think his sorry indicates he's learned the lesson. If he does it again, I'd ground him by not letting him go out with friends and in your situation I might make it for a couple of weeks. If he reacts negatively I might add another week, explaining that this is the consequences for not handling the first grounding appropriately. THen I'd explain what the appropriate behavior would be.
Not being able to go out is a natural consequence to going out without permission. Taking away the phone and computer makes no sense. I would have rules for using the phone and computer already and might tighten those a bit. For example if he can talk on the phone whenever he wants, I might limit the time he can spend on the phone so that he can spend more time with the family. Then make the family time enjoyable.
How do we feel when our boss orders us instead of asking us to do something. I feel like he's being authoritarian and taking away my power to decide that I will co-operate. I feel that he is not accepting that I'm a person with feelings who is more than willing to co-operate when my co-operation is appreciated. I give people a lot of praise knowing that the praise will help them to feel good about themselves and thus be more willing to be co-operative.
I'm a retired police officer. As you know a police organization is quasi-military tho it's getting to be less so in some jurisdiction. We said, "orders is orders" and did what we're told. However most of my supervisors asked me to do things, assuming I would do them. With the exception of one lieutenant I rarely received "orders." I was a part of a team who learned when I could have input and when it wasn't appropriate. Team work works best for a teenager. It appears to me that he either didn't understand that 15 was the age to babysit or he did but wanted to exercise some control over his own life. When you take away all control for a month he doesn't learn about how to be a part of a team. He learns that he must obey orders. However, because he is a teen, he will withdraw and/or act out so that he will feel that he does have some control. He's willing to accept the consequences, i.e. be grounded some more. Life becomes a subtle or open battle of wills.
Some of this especially regarding orders and asking may be based on my own biases and not on your situation. Rules are ways of having boundaries and getting co-operation especiall if the 16 yo is able to have some input into the rules concerning him. Rules are not orders.
Unless your son is usually out of line and unco-operative I think that a month, even if it's just the weekends of having no contact with friends, being banned from the phone and the computer is too harsh. Discipline should be progressive. In most jobs we first get a verbal warning, than a letter in our file, then time off without pay and finally losing our job if we continue with the same offense. Being disrespectful in different situations is the same offense. Leaving the younger kids alone more than once is the same offense. Not following the rules, that are in writing and well understood, is the same offense even tho the situations are different. Was this the first time he left the younger kids alone? Was he sorry? Does he understand that you will not tolerate that action again? Then does he really need to be grounded from friend, phone, and computer. That really seems like overkill to me.
If you want to change the consequence you can without losing your authority by explaining that you made that decision while you were angry and that now that you've thought about it you'd like to reconsider. What you decided upon reconsidering would depend a lot on his attitude. Co-operation is the goal. Learning to make appropriate decisions is the goal. By admitting your anger you will be showing him that decisions made in anger are frequently not appropriate ones.