In the last missions of dbh, ssj4 xeno gohan's power was shown to be comparable to the likes of ssj4 xeno gogeta.
Ssj4 il a potential unlock by the,words of gt perfect files, is a form that brings out every last bit of latent power of the user, so logically the multiplier is different for everyone ( but still, heroes forces us to think that Goku and vegeta do have the same multiplier), and we know that gohan has much more latent power than his dad. In fact, in the video game xeno gohan's ssj4 is much more like broly, his strength is ever growing.
Anyway, in dbh ssj4 gohan has always been powerful :
Dbh s17 arc (gt) gohan turns ssj4 and he's a more than valuable help to Goku and vegeta against s17 (#18 absorbed)
Dbh shadow Dragons saga (gt) ssj4 broly appears, and ssj4 gohan fights better than his dad and vegeta against him