Dragon Ball - DBVS (Doujinshi) - Vol. 3 - It's Finally Settled!! The Ultimate Evolution, Super Saiyan 4 God Rushes!!

[MangaDex]

 

[PillarMan-FD]

I feel like things make a little more sense with how Goku Black attained Super Saiyan Rose. There was little explanation as to how he got stronger in the short amount of time between his first fight with Goku in the main timeline, and when Goku, Vegeta, and Trunks traveled to the future timeline. Black meeting Goku in the GT timeline, and fighting the likes of SS4G in it's full glory during that time certainly provided him with ample opportunity to gain more strength and prospective of Goku's abilities, no matter which form he takes.