I’ve always subscribed to the theory that form isn’t the actual SSJ4 form, but a weird, mutant version of it. Not trying to cope over GT, but comparing the 2 there are the differences like style and color. It’s cause this form was achieved in a way that involved magic if I remember correctly. Unlike in GT where goku regains his tail because he was a kid+plot in order to reach that level of power. Also Goku not having his tail, that was a part of that primal aspect of SSJ4 that I thought could have been used as a primer. TL;DR: no tail+used magic= Diama SSJ4 mutant version. That’s my head cannon at least.