Difference between SSJ4/Ultimate Form...

[Southern Gothic]

As it pertains to potential unlock? Which unlock do you consider to be greater? And is it theoretically possible to stack one on top of the other, or do they individually max out the potential so no further gains are possible at that time?

 

[Captain Cadaver]

Elder Kaioshin said he draws a user's power far beyond their natural limits, whereas SS4 is stated to draw a Saiyan's power to their limits.

Ultimate >> SS4. I'd say the difference between them as it pertains to the user's current limits would be about 1.5-2x.
From what GT implied, Ultimate is just a power boost and the new base form of a user in it's continuity, plus the limits of GT characters are already far beyond their manga canon selves. That said, I believe you could potentially stack SS4 on top of ultimate if the form was gained after the ritual, given the ritual opens a user's limits to a new plateau (eg. Vegeta believing Gohan, Goten and Trunks could beat Super Yi Xing Long with enough training when Beyond Limits SS4 Goku could only touch him with Ryu-ken).

 

[withheldforprivacy]

Uncle Withheld has explained in the past the difference between current potential and
lifelong potential.

 

[Southern Gothic]

I searched "Chester the Molester" and came up with nothing, so I'll have to get my answers here.