My daughter is 4.3 and I always buy at least the next size up, eg. she wears 5-6, or even 6 at the moment. Depending on the make, I have even got a couple of bits aged 6-7 to try and get some wear out of them and no way do they look ludicrous or anything.
She's always been tall, but isn't overweight. From what I can remember while she was still getting weighed regularly as a baby, she was always in the top 10% for her age ..... someone has to be !
Like Pinkteddy, I find the main problem is finding age-relevant stuff. I do not want to dress her in glittery cerise/black lurex things with "sassy" slogans which seem to be so popular for the next age group. You can find stuff but it means a bit more searching about and I daren't usually buy mail order because I always want to try stuff on first.
IME, I've always found that Next tops usually come up a bit short (for the stated age group) whereas their trousers are more generous (or maybe my daughter has a long torso and short legs ?). M&S seems quite generously sized but they're that bit more expensive, don't have a huge choice and I don't like a lot of their range. Adams come up very small too. H&M do a good kids' range - some of it noticeably "different" IMO, but again, I always have to get at least the next age.