I was thinking that would be nice to have the possibility to create mp4 as container of the AAC created by iTunes, since Apple codec seems to be the best so far and the only two limitations that have are the missing of HE (fortunately this issue will be corrected soon from what I've read), and gapless playback (only possible with MP4 container)...Thanx for Your answers

If you encode the file in Quicktime you'll get a MP4.

Thanks, but how can I do it and...will the MP4 be gapless?

I managed to do the mp4 but I've discovered it's not gapless (the number of samples is not a multiple of 588 when encoding from a wav track extracted from a CD...) I think the only solution would be to shift to Nero AAC or FAAC...

Indeed, gapless encoding is not yet implemented in iTunes.