Starling, been thinking. Don't like to suggest telling white lies to children as a rule, but in this case it might effect a compromise between self expression and protection against teasing:
How about suggesting to your friend: Saying Barbie trainers 'only come in girls sizes', but you can find a pair of Barbie socks that would fit. Socks would be a bit more discreet (and cheap) than trainers - and you could tuck a spare pair of ordinary socks into his school bag, so if he was teased, he could, if he wanted to, change. I used to tell my son that Mothercare had a special 'sports' section when at 5 years, he had a worrying tendency to request socks and 'T' shirts from expensive sports shops.
I think Emmams idea about a walk past the school is a very good one, too. I'd definitely do that pronto.
On the other hand, you don't want to imply it's silly to like girls things. Robinw, how sad that that the little boys your daughter knew were not allowed to play with dolls houses. Glad you found a way round it!
For a 4 year old, it's an eternity from now to September, Barbie might be usurped by Action man -who knows. I'd be inclined to go with the flow for now and buy the odd bit of Barbie, but also get him together with lots of school-aged boys and girls eg in park playgrounds,so he gets a general feel for what's in and what's out.