Assuming your baby is not 28 pounds, , I'm just curious, what made you decide to try the size 4 diapers? Did the size 3 diapers clearly not fit? The weight ranges overlap so much, with size 3 going from 16 to 28 pounds, and size 4 from 22 to 37 pounds, so I'm not sure when the best time is to switch, especially since the size 4 diapers cost more per diaper.
I've never tried a size 4 diaper on Keira who I would guess is around 22 pounds, and don't know if it's worth buying some to give them a try.
Re: If your LO wears size 4 diapers
His big brothers wears a size 4 (no, he is not potty trained yet ) so it is easier to have them just wear the same diapers. Connor was actually in a 3 for the longest time, but as soon as he hit 29 lb and Ryan hit 22 lb we moved them up to the 4s.
You get more in the smaller sizes so I would wait.
DD -- 5YO
DS -- 3YO