Hi Kristin..Wow, 4 weeks, that is seriously young to be separated from mama and his litter-mates. The 8-10 week rule isn’t just because of nursing. During this time, there is important social interaction that takes place within the mini pack and between litter-mates.
I would strongly suggest milk replacement if you aren’t using it already..He still needs the easily broken down nutrients of milk. NOT and never cows milk though. You can find replacement milk at any petco, persmart, walmart, or local pet shop or, veterinarian. You can use a regular baby bottle as well with a premie/newborn size clear nipple. Good luck and congrats.