Separated, not divorced. Is it wrong to not allow my husband to take my son 4 hours away?

If you have a piece of paper written where you have "50/50 shared custody," this will not hold up in court if he decides to leave town with your child. LEGAL doentation is needed to do anything or to stop him from taking the child. Get in the courtroom and get a legal parenting plan and custody arrangement set up to protect all of you.