When my niece was 4, she could dress herself with minimal supervision, she can play by herself for a few minutes,she picks up her toys, temper tantrums were almost nonexistent.

Now ss4 is how can I put this nicely...lazy and a spoiled brat!
You can't leave him alone for a minute, when I'm home alone with them and go the bathroom, he follows me in there or stands at the door knocking till I come out, he absolutely can't(or won't) play by himself, when the ssons are over(we have them 50%), SO and I can't hold a conversation until after they go to bed because ss4 is always "daddy, daddy, daddy" or "sm, sm, sm" the boy wants attention ALL the time! He throws tantrums at the drop of hat, wants either me or SO to pick him up all the time! He baby talks, spreads his toys all over the room and won't pick them up when ask. Most of the time he won't respond when you call his name (no he's not deaf, SO had his hearing testing in December).

I feel like he's so immature for a 4 year old! BM and SO didn't start potty training him until after his 3rd birthday and it took 10 months for him to finally be potty trained and now he won't pee in his undewear but he poops in it all the time. Bedtimes are a nightmare because he won't sleep unless you lay down with him and it takes on average 45 minutes for him to fall asleep.

ss3 on the other hand, can play by himself, he's not constantly interupting adult conversations, picks up the toys/books when you ask him to.

I've pointed out to SO how needy and immature SS4 is compared to his younger brother, and SO's response is always "SS4 had a hard life! BM breastfed him until the day ss3 was born and then just yanked him off the boobs, BM stopped showing affection to ss4 once ss3 was born..." The ironic thing is, whenever ss4 is held accountable for his actions at our house, he cries and says he wants to go to BM. I feel like SO is contributing to ss4 acting like a brat by feeling sorry for him. He lets him get away with things because he feels sorry for him for "having to live in two different houses" Even MIL told SO that ss4 needs to be treated more age appropriate and not like a baby. SO seem to expect more from ss3 than ss4. Somtimes ss3 would get into trouble for the excact same behavior ss4 got away with! I've called SO off on his tendency to look the other way when ss4 misbehaves but punishes ss3 when he's only copying ss4's behavior.

I adore both my ssons, they're not too bad but to be honest, ss4's behavior is really starting to annoyed me to the point that I make plans with friends or volunteer to work on the weekends we have them. In public I physically distance myself from SO and ss4 whenever ss4 acts like a brat or a lunitic.