My issue with Super Saiyan 4 is how every character who has it ends up sharing too many design elements.
They've all got that extra hair that drapes over their shoulders, and they lose their shirt in the process.
Which doesnt sound like much, but when said characters all have the same face, torso, and very similar hair, they just end up blending together. Hell, Im assuming the reason why Gogeta had red hair was purely because he would be indistinguishable from Goku otherwise.
I mean, Super Saiyan forces every character to have blonde spikey hair, but its less of an issue than 4. Especially in the manga.