How do you discipline a 3-4 year old?

We are having so many behaviors and I’m finding this age really tough. They thrive on social interactions at this age but he isnt going to preschool for a couple months atleast and who knows with this virus. We are finally over the hurdle of him getting used to his baby sister (she is 9 months now)

He won’t play on his own at all... even after hours of playing with him then he just screams and starts to slowly misbehave if I have to do something with the baby or anything. Constantly calling mama... then he is demanding I do things for him (we tell him constantly to try to do it himself because he can then usually turns into a screaming meltdown) and acting helpless. He is just not stimulated enough... but nothing I can do. I feel he is between ages in terms of toys too.

Anyways we set boundaries and are consistent (no climbing on the couch, no climbing in babies play pen) we take away privileges... treats and going places tho there is such little to do right now. I also feel taking his treat away because it wouldn’t be until later... it’s too much time between.

I don’t know overall he is pretty good considering the circumstances but we are dealing with a lot of screaming and demanding behaviors. I just feel bad for him too but my husband seems to think we need to discipline him more.

I just want to know what you all do... especially during these times our kids are home with us more than usual