You may not think this is all effecting him because he is only 4yrs old, but it is. He may not understand what is going on but he knows mommy and daddy aren't happy and he just moved in with grandma. He can feel the stress and sadness coming from his mom too. His mom (and dad if possible) need to sit him down and explain to him in simple terms what is going on between them and that no matter what they both love him very much. He needs to be allowed to ask questions and maybe even learn some new words to express how he feels about the whole thing. Don't just brush off his feelings because he is a kid. My daughter was 3yrs old (almost 4) when I moved from Indiana to SC. She left everything she knew-home, babysitter, grandma, her dad, etc. There has to be an adjustment period for a child.
Also, take away anything stimulate to him about an hour before bed time. NO TV, rough housing, tickling, etc. That hour before bed should be used to get cleaned up, put on pj, brush teeth, maybe even pick out clothes for the next day, read a book or two with you or mom.
Good luck and tell mom to slow down.
S.