My 4 a Half Yr Old Sucking His Thumb

My son sucked his thumb from birth (and probably before) until he was about 7.
There was no stopping him.
Starting in pre-school we told him the thumb was only for nap time and bed time.
School kept him so busy it wasn't a hard thing to do plus there were plenty of other kids that needed reminding to keep the thumbs out of mouths except for naps.
My guess as to why your son just started this is he's copying the baby for attention or he's feeling stressed about something and this is a self soothing thing.

to stop my son from thumb sucking we put socks on his hands.

Both my kids sucked their thumb from birth until about age 7. Once the permanent teeth come in the dentist wants the thumb sucking to stop. We used the nail polish that is used for stopping nail biting (bought it at CVS). It is very bitter.
But for your son it is probably just a time for regression since your youngest is getting a lot of attention and is probably beginning to walk and get into his stuff. I would let a 4 yr. old have this self soothing technique or else try to steer him to other self soothing things, like a favorite stuffed animal to sleep with, or combing his hair with one of those super soft baby brushes. And perhaps let him have an area that is just for "big boys, no babies allowed" so he can play in peace for a while without having to share everything. Plus make sure he gets a lot of cuddle time with your still, or else with hubbie. Divide and conquer worked for us. Good luck.

Sounds like just a little regression. Copying what little bro is doing because he saw it as 'cute'. He may also be trying to experience the world as his little bro does so he can better understand/relate to his challenges.

I'd be inclinded NOT to focus any attention on it. When the 3 of you are playing together, you can encourage him to show little bro how a big boy does this or that. The more you can positiviely encourage him to display correct hand postures the less likely any of this will become a habit. And, if it does become a habit, peer pressure will likely snap him out of it really fast come kindergarten next year! I was a huge thumb sucker and nothing my mom tried - including mittens, gloves, nasty tasting polish, or threatening to cut my thumb off on the butchers block - worked. What worked? Kindergarten. NO WAY i was going to let my peers see me do that! (BTW, I highly, highly, suggest you do not try the butchers block threat. Scared me to pieces for years and years to come. I mean, WHAT was she thinking?? Holding my little thumb down like that and holding a knife over it? Really???? ~shake my head. again.~)

My daughter was a thumb sucker. My husband hated it. However, the more he threatened her with punishing her, the more she felt the need to comfort herself.

She only did it before going to sleep and she would suck her left thumb and use her right hand to rub her left elbow. It was a comfort thing. If she pulled the blankets up over her head, I knew what she was doing. She was sucking her thumb "in private".

Anyway, I think that what your son is doing is mimicking what his baby brother is doing. I don't think he needs medical help at all.

He likely longs for the days when HE was the baby. He may be not even conscious of his actions.

Give him plenty of attention, let him know he is loved and he will always be your first baby. Let him know that you are so proud of the "big boy" he is growing up to be. Enlist his help with doing "big boy" things that his baby brother can't do like baking cookies or decorating cupcakes.

He could be feeling like everything is about the baby.
Let him know how important and special he is for being a big brother.
Make time for him alone as much as you can.
Let him know he doesn't need to be a "baby" in order to receive your love and attention.

Look at things from his point of view and I'm sure he'll be fine.

Just my opinion and best wishes.

We tried everything, the nail polish, mittens, etc. Nothing worked. So I took my daughter's favorite blanket away. Then she stopped. It was mean but it was the only thing that worked. She was 4 or 5 and only did it when she was tired and holding that special blanket.
She is 16 and still has that blanket and if she sleeps away from home she keeps it in her pillowcase, lol!

Whenever you see him sucking, don't tell him to "stop sucking." That's negative and punishing.

Redirect instead. Ask him to pick something up and hold it, hand something to you (be a helper), hug a pillow as hard as he can, etc. Be creative.

That way you're not punishing or being negative. By redirecting, you're transforming an undesired behavior into a desired, positive behavior for which you can praise him!

Medical help is most likely not needed here. Try this and he'll grow out of it. Good luck!