SSJG's power is too flexible to be nailed down. If you watched Battle of Gods you see the previous SSJG get beaten down by a normal Oozaru. That alone tells us that the power SSJG gives is entirely dependent on the Saiyans involved in the ritual. SSJG Goku was so strong due to the strength of the people involved in the ritual as well as being so strong himself.
Now, the GT Perfect Files imply SSJ4 Goku is either equal to if not stronger than Super Vegito. SSJG Goku along with Beerus are confirmed to be stronger than Super Vegito.
For the sake of argument, well say both SSJ4 Goku and SSJG Goku are stronger than Super Vegito.
Also, and im not sure if you heard this yet, but in a interview I wanna say..2-3 months back, Toriyama said that during Goku's battle with Beerus, he absorbed SSJG's power into his own body, basically meaning he no longer needs to transform to use its power. Essentially its now Base Goku = SSJG Goku at this point.
Now with that info, if SSJ4 Goku fought SSJG Goku, SSJ4 Goku is going to win hands down due to SSJ4 Goku already having SSJG's power inside him. All SSJ4 is doing is multiplying that power by 4000.