1 thing that both GT and Super did right.

[Let's Go Fearless!]

Is to have SSjinGod/Blue and SSjin4 exclusive to the full blood Saiyans. What do you say boys?

 

[Boo Brand Milk]

 

[Captain Cadaver]

I disagree, since it just made it more apparent that Goku and Vegeta would get special privileges over the other cast members who would find it even harder to retain plot relevance. Neither continuity's new forms are even stated to only be exclusive to full blooded Saiyans iirc (not to mention spinoff material such as Heroes blatantly contradicting that standpoint), making the decision completely unjustified with how easy the forms could be obtained under the right circumstances.

 

[Papasmurf]

Closest thing to confirmation on SSJ4 being exclusive to pure Saiyans was Vegeta assuming his inability to go SSJ4 when he went Golden Oozaru while under Baby's possession was due to him being part Tsufurujin.

 

[Captain Cadaver]

I wouldn't say that's much confirmation though. Baby was a parasitic organism who wasn't implied to be capable of actively breeding with other races, so far different from human-Saiyan hybrids which were biologically natural.