Size 4 to size 5: when did you transition?

[Snow mom]

DS is doing well at potty training and I have to make a decision on diapers. I ended up with way too many diapers stashed away based on good sales. He's wearing a diaper at nap and at bedtime but I use the same one if he happens to still be dry when he wakes. I'm running out of the current box and need to decide if I should move up sizes or not. I could either open a box of size 4 (152 or 192 count, so a couple month supply) or a box of size 5. I know it's technically possible to not use up a box of diapers but I'd rather try to resell the unopened boxes than have him seriously outgrow the diapers we are using before we finish the case. DS is 2.5 years, 28 lbs, and tall and lean. It would be helpful to know when others moved to size 5. He isn't a heavy wetter overnight so I'm mainly worried about the diapers physically getting too small for him. I don't think DD ever made it into size 5 so it's unfamiliar territory for me.

ETA: if it makes any difference I have two large cases each of size 4, size 5, and size 3-4t easy ups.