The truth is there is no official reference known about SSG power levels yet.
Apart from that, I think SSJ4 is stronger than SSG, even knowing SSJ4 and all the GT content doesn't belong to the Dragon Ball story anymore.
Now, we've got to think about the new developments on Dragon Ball Super
, where both Goku and Vegeta reached the SSGSS
form, which certainly surpasses the SSJ4.
Having that in consideration, we can go even further and think about the latest achievement of Goku, reaching the SSGSS Kaio-Ken x10
form, that by all means, is the ultimate Saiyan form until now.
We don't know if there will be any stronger variation but, for now, we should have in consideration this form that Goku accomplished.
Conclusion: SSGSS Kaio-Ken x10 is clearly stronger than SSJ4.