Two New Babies, One Sleeps Constantly

Just as the title says, I have two new babies that are about four weeks old. One is always up exploring and doing his thang, the other one is awake for maybe an hour or two a day. When he is actually awake, he doesn't seem interested in playing or climbing and just seems unenthusiastic in general, though he's still eating and drinking. Last night, I decided to wake him up and take him out of the cage to see if I could get him moving and after maybe a minute of curiosity, he just fell asleep on me. I've only been a rat owner for five months so I'm still pretty new to their behavior, has anyone else experienced this with their babies?

Does he seem to feel unwell? It is normal for babies to sleep a lot, but if he is lethargic he may be feeling sick.

It sounds like he is ill so a vet would be a good idea.
Good that he is eating and drinking
What are they eating and do you have a water dish in their cage?
How is his breathing?

btw, as you likely know, at 4 weeks they should still be with their mama