This is a tough one. If you had three it would be a lot easier, with 4 it's going to be a challenge. If you had 3 you could leave the little one with mom all the time, and rotate out the other two, but with four you can't do that.
I would start immediately supplementing the little one. If he has no energy, he's already on a downhill slope. I would feed him every 2 hours around the clock, then I would pull the other three out of the cage with mom every 2 hours and give him some time with mom by himself, so she can take care of his potty needs as well as giving them mom/kit bonding time and give him a chance to nurse. Feed him after he comes out from mom, not before he goes in.
Put the little guy in a baby safe carrier by himself with a heating pad underneath on low and a stuffie to cuddle up with or a sock with rice in it. Just something to snuggle against. Stimulate his genital and rectal area after you feed him with a very slightly damp, warm, wash cloth or cotton ball.
I have to tell you though, if he's already slow and not walking well, getting beat up and not nursing, his chances are very slim that he'll make it. That may all change with the hand feeding, but just be prepared for the worst.
