Gotenks ought to be able to handle him without too much difficulty. Logic would dictate that Ultimate Gohan >> Gohan's Natural Limits >>/>>> Goku's Limits, whilst SS3 Gotenks is suggested to not be massively weaker than Gohan.
In n my opinion, most of Goku's GT power doesn't come from his potential or transformations. Rather, its because his base form is so damn high that it makes any increases extremely crazy.
Giving him SSJ4 in Z doesnt hold as much weight due to this.
It says it brings a user to their utmost limits. Not only is Gohan's limits already much higher than Goku's, but Rou Kaioshin brought Gohan fa~ar beyond his limits. Gotenks isn't far off from Gotenks, there's no way a Z SS4 Goku could come close to either of them.
Even if he gets the same boost he got from it in GT, the farthest I could possibly see him getting is SSJ Gotenks. If it's just a potential unlock, I doubt he gains anything significant, in the same ballpark as the majin boost for Vegeta. I honestly think he was at his full potential at that point.
Pre-unlock Gohan's limits surpass this version of Goku's by quite a bit, and post-unlock Gohan widens this potential gap a notable amount further. Gotenks isn't too far off from post-unlock Gohan, so SSJ4 Goku would get straight up violated here.
If SSJ4 is a multiplier: Goku slaughters.
If SSJ4 is potential unlock: Goku gets slaughtered, if we consider only the potential he should logically have in Z canonicity.
In a sense, all of the SSJ forms are potential unlock. I mean, in order to unlock the SSJ, a Saiyan should have it in him in the first place. All the Saiyan needs is a push, if you know what I mean
Anyways, SSJ4 Goku should win in my opinion, as I don't have Gotenks that much stronger than him.
I go with SSJ4 being a multiplier, and even with that multiplier I still wouldn't have Goku being strong enough to beat Gotenks unless he whips out the Ryu-Ken so without that I give this to Gotenks.