I bought these for my daughter who is just over 2 years old.  The length is good on her and the rib-knit material is snug enough even though this is 4T. She's on the tall side ~75th percentile for height and average weight, but I imagine that this would be short for a 4year old.  Previously bought the Hanes 2-3T size and the material was thinner and much shorter + shrank in the wash (it isn't fair to compare the size since the Hanes was supposed to be smaller).  I will continue to buy these but size up considerably.
The hem is folded over and sewn instead of just having the cut end of the fabric finished off to prevent fraying (like Hanes) so is a little bulky at the hem and it causes the hem to fold over (does not cause a problem for us, but may be annoying for some).