1) if you are only at 4, things can get dicey quick -- especially in the age of the portal. A late transfer or an injury (or two) can make you vulnerable fast. One of ND's top 2 RBs (projecting) for '22 doesn't seem like a "20+ carry per game" type player (Tyree). And two of the other four have little (Estime) to no (Price) experience. I get your point, but I also see why ND would want a 5th RB.
2) if you believe the RB you are going after is going to be star, you go after him regardless. IMO. Paul is only rated as a low 3*, but his recruitment has exploded recently. He had a monstrous SR season (nearly 3000 yards rushing) and has started to get recognition. He was named Texas POY this season, after being relatively unknown going into the season. Right now many top programs are after him, and pushing hard to get him.
I'd suggest watching Paul's SR highlights. There is a lot to like there. And he looks like a solid, sturdy guy despite the quickness he shows on film. He's listed at 5'10 221 at Rivals and 5'11 220 at 247. I, personally, would be thrilled if ND landed him. But I think it is a longshot.
Link: https://247sports.com/college/oregon/Article/Andrew-Paul-recruiting-Clemson-Michigan-Notre-Dame-Oregon-180885895/