Finding a 50-something 520 will be super hard. Finding a fat 24 in the 507 flavor is incredibly easy.
Yes, an s4 is a 584, aka 650b, 26x1 1/2", 27.5", and probably other bone-stupid aliases. I like the ETRTO or ISO sizing, where you express the tire size by the most important measurement: the diameter where the bead sits on the rim. Ppl go looking for a "27.5" tire, there's a good chance they'll come home with a 630 ("27 inch") instead of a 584. Ppl go looking for a 650b, they might mess up and get a 650A (590) or a 650C (571) instead. Millimeters are universal; the fact that we've come up with like 4 names for a 584 that have the potential to cause confusion whereas a 584 is really pretty cut-n-dry, well, that's just frustrating. Is it a 27.5", or a 26x1 1/2", or a 650, or a 4? It's not any of those, at all. I mean, it MIGHT be a 27.5 if the tire is fat, or it MIGHT be a 26x1 1/2 or a 650 if it's kinda skinny.... but having a standard way of discussing this will save these threads and the confusion...