Hi, I'm new to the forum and just saw your post and thought I'd reply even though it's been a while since you posted it.
I have a 4 year old boy with two older sisters. While he's never said anything about wanting to be a girl he's not been adverse to dressing up in their old dress ups, or wearing necklaces, rings or a flower clip in his hair or whatever.
I'd say it's not unusual. He may indeed get teased once he starts school and starts making new friends, but if he's confident within himself he won't care. There will probably be a few kids in his class that are like-minded or not judgemental and he'll make friends with those kids-or, play with the girls.
His own confidene and sense of self will determine how he deals with his curious or unkind peers. Is he in Kinder? It's good to have him in a inder where a god percentage of the ids will be attending his same primary school. If you can get a couple of kids he'll be in prep with, invite them over or have play-date ALOT.
Hopefully at least one will 'stick' and he'l have a best friend early. Non competitive sports (my son wants to do gymnastics) are great confidence builders too, and a little 'leg up' with his letters and numbers before starting school will make him feel good about himself acedemicly also.
Once a child feels like they are 'IT, and a bit' and have plenty of love and support, and a couple of good friends-teasing can't touch them.