Welcome to healthcare magic.
For a 4 mth baby its best to sleep on his back i.e. supine. But some babies prefer to sleep on their tummies or side no matter how much we direct them .Its a normal occurence and you need not worry for that.Unless grossly different, mild difference in body fat of the two hands is normal and wil be equal as they grow.It might depend on variable blood supply n usage..If you still feel concerned or find any more signs you should visit your pediatrician
for a check up .hope I have answered your query.thanks