Fearful 4 Year Old

It is just an age-stage phase.
Any kind of "separation anxiety".... will STILL occur at various ages. BUT... it is manifested and expressed differently, per age.
Now that he is 4, he can say and express his feelings. Which is good and shows good development for his age. Versus, a 1 or 2 year old may be feeling that, but per those ages they CANNOT "express" it verbally or emotionally, hence at those ages they tantrum more.

Your son is normal.
And fine.
Don't worry.
Even older children get like that at times.
Just teach him that he CAN tell you how he feels and its okay. A boy... needs to learn that and know that it is okay. It is better than having a pent up boy... who then later becomes a pent-up non-expressive older son or Man.
Nurture his expressions to you. At this age they are not fully developed yet emotionally nor know the "names" of feelings. Teach him that.
My son is now 5... and since 2 years old I taught him those things I mentioned above. He is now very articulate about expressing himself, knows his feelings, trusts his gut, importantly.... he KNOWS that he can convey these things to us. Without critique. He is thus, very self-assured. But sure, he gets more "clingy" with me sometimes. It is fine.
Because I know one day, he will not 'need' me... and will not even want to be near Mommy.

Again, Separation Anxiety... occurs at ALL ages. It is not just in babies or toddlers. But per age it is just manifested differently.

The book "The Kissing Hand" is great. I would get it.
Any bookstore will have it.

Instead of trying to "convince" him to feel the opposite... just confirm that you understand how he feels... and allow him to express things to you. Because... as he gets older, you WILL want your son to feel comfortable about telling you things. Some kids, DO NOT EVER, tell their Mommies anything nor about their feelings or feelings of separation anxiety.
It is good... your son is being "eloquent" per his age and communication level, about how he feels.
YOU... are his soft place to fall.
YOU are his Mommy.
He is fine.
My son gets like that too. A son is bonded to his Mommy, differently than with a Dad.
I have a daughter, and she is bonded to me closely as well, but differently than my son.

And sure, a child needs time to get used to different routines. And school. And all that stuff. For a little child... it is a BIG world out there. Much to digest. And everyday, varies. Its okay.
Just let him confide in you.
Assure him that its okay.
Sometimes, all a child needs... is empathy. And that the Mom understands. They are "venting" in a sense, to you. As we women do with our family or friends. Sometimes we just have to vent... and express it then we feel better.... once we know, that our Mom understands, us.

A 4 year old, is never spot-on in terms of listening or doing things ON target all the time. It is hard "work" for them in school. They are not robots. Your son seems fine and is his age.
Even 5 or 6 or 7 year olds... do not behave perfectly nor listen like statues all the time. It is them learning.
And little 4 year olds, have a hard time being still as a statue or not moving. It is normal.
"Rewards" will not always work... because, the age and maturity development, and expectations of a child, has to be in line with the age stage.

Again, your child seems normal and fine to me.
I have a boy and girl.
And at those ages they get like that.
They are not.... self-reliant yet at that age, nor fully developed in terms of deductive or inductive reasoning. They don't even have the ability to fathom or analyze "feelings" yet. Nor the everyday expectations that are upon them.

And per night time... yes, they get lonely and want you there. "Fears" of nighttime.... occur at these ages and is normal. Normal. Normal.
I was like that too as a child. But I am a fully independent person. But I loved my parents so much. I "missed" them when I went to bed. They knew I would outgrow it one day. And I did. That is childhood.
They are little.
Also, "fears" are developmental based and age related. EVEN 7 or 8 or 9 year olds, get afraid of the dark for example. It is just generalized fears, as most kids in childhood, experience.

I went through this with both of mine at age 4 and it came on suddenly. My daughter's had a reason: her best friend moved away and her teacher moved to a new room at the same time. My son didn't have any real reason, he loved, loved, loved school. One night I helped my SIL deliver her baby and the next morning he refused to go to school. He said he wanted to stay home and play Wii instead. I dragged him kicking and screaming into school. After 20 minutes the director pryed him off of me and took him to class and she said he was fine as soon as I left. When he got home I took his Wii remotes away. For a month he couldn't play Wii in the mornings. Problem solved. I still don't know if it was me being away that night or his desire to play his games but thankfully it was over quickly. It sounds like your son had some great time with mommy and thinks everyday will be that way. If you can, find some non-school time to spend some extra time with him and tell him that on school mornings it is pretty not fun.

He loves you to pieces and had a great time with you on snow day.
This is him telling you "I love you and need you mom".
Working outside of the home is a tough task. When you get home you have to hang with your kids and let them FEEL that security. You have two jobs when you are a work out of home mom. Kids love their parents and NEED their undivided attention for a time each day.
A child with good esteem is a child that is close to his parents.
Close meaning a mom and dad that treats them like they are important and not a nuisance or "job".

They go through this every once in a while.. He may have gotten the idea, your day is like the snow day.. You work a little, go swimming and go out to lunch everyday! Ha!

Also the older they get the more they are beginning to understand they are not babies anymore and are expected to behave at all times. Instead of tantrums, they have melt downs. So many feelings and emotions hit them all at once.

Do Not make a big deal out of theis. The secret will be to keep him on his schedule and talk about the positive things that he is "getting to do at school".

"The teacher says she misses you on the weekends.

" I heard you had fun with Charlie on the playground today."
"Ooo, this art project looks cool, I wish I could do art projects at work. I had to sit in a boring meeting all day."

"Sorry the weekends can be kind of boring without your school friends. But I have to clean the house on the weekends."

"We have to go to the grocery store on the weekends because I work all week." " I bet you wish you could be playing at school."

Don't go overboard, but you get the idea.. Remind him what he DOES like about school. And how proud you are of all of the things he is learning at school.

I would not rule out the fact that there is a new aide in the classroom.

I was just watching a utube video tonight, where a 2nd grader's parent had been told that he was having behavoral problems. He sent the kid to school with a recorder going in his pocket. And all kinds of mean spirited stuff was being said to the kids. I am not saying that this is what is happening to you, but I wouldnt rule out that 'new' situation at school totally until you know for sure what the issue is.

And then there are some kids that are just my timid or fearful, and others that are more bold. I have a very loveable, but was very fearful, son. (he still wont go on amusement rides at 11.) And I have a very bold daughter, that nothing scares her.

Yes. My youngest refused to go to the bathroom - or anywhere else - alone starting at around 5 because of monsters. The only thing that seemed to help was simply accepting her fears and "mirroring" them: I would say things like "You're afraid, aren't you? I bet that doesn't feel good." and she seemed to calm down a bit.
Time, of course, was the ultimate helper, but that is hard to wait around on.