Does digitizing Super 8 film at 4K resolution make any sense?
Calling super-8 720p is confusing. There are no rows of pixels, only grains of emulsion, scattered in a random pattern on the base.
That said, film does have an upper bound on its resolving power, in other words, the smallest detail it can reproduce. Kodachrome could resolve 96-135 lines per millimeter, which can be roughly converted to 180-270 pixels per mm (it takes two pixels to resolve a line). A super-8  frame is 3.3mm high, so the vertical resolution is somewhere between 540 and 810 pixels. So as a back-of the envelope description of its resolving power 720p is a good approximation.
So if you capture super-8 in HD you will be getting enough resolution in your digital file that all of the image that is present on the super-8 frame will be preserved. Higher resolution will just give you more detail of the grain. It won't look worse, but it won't look a whole lot better.
What I would be more concerned about was the fidelity of the colour, and the quality of the transfer. A well transferred, lower resolution transfer will beat a poorly done higher resolution transfer any day. You can just project the film on a wall and film it, but it's probably going to look dreadful.