From the pictures, the bun does look a bit older than 4 weeks. Closer to 6-8 weeks, which is much better for the little guy.
Sexing baby rabbits can be very hard, and it is not unusual to be absolutely convinced about the sex of your bunny, only to have that change when they get older. Some breeders have had a bunny enter into a show, win as a doe and then next week end be disqualified for being a buck entered in a doe category. Here, we call this getting a visit from the "Gender Fairy".
When I have young bunnies I normally check once a week until they go in for a spay/neuter. I can normally get the sex correct from about 8-10 weeks.
--Dawn