I have had a lot of trouble finding underwear that fits my 4.5 year old son.  He generally wears 4T pants and shorts though the waists can be too big, and the waists of 4T pajamas are always too big, but for some reason underwear brands seem to run small and cut into his waist.

I already had one pack of these and they fit him well and I liked the soft waistband, but I still wanted to find a less expensive brand.  Eventually it was costing me more money to keep buying underwear that did not end up fitting, so I ordered another pack of these in 4T.  Overall I am happy but each short in the pack is has a different waist size and one is much smaller than the others.  I hope they will stretch out to the right size with time.