It kind of sounds like the child is overwhelmed by the sudden change in life. And, if his sibling is not coping well either then it would make his adjustment harder. At 4yrs old if you are stressed about something, having another child bawling and stressed about it doesn't help you build confidence!

I think that as hard as it is for a 12month old to adjust to daycare it can be even harder for an older child. 12 months old don't really have a clue whats going on and just get used to the new norm...4yr olds can sit there all day remembering what their life was and comparing and grieving the loss of old ways etc. While it is harder to have patience for them at that age we still need to be put in their shoes and figure out what they might be reacting too and try to work with it.

I'd suggest identifying the emotions. "it's hard to not be at home anymore, it's hard to not have mom with you all day" etc etc. Give warnings before you transition. Transitions are really hard on kids that are not settled and feeling safe and confident. Give lots of warnings as to what is to come so they aren't taken by surprise.