x4 seems to be more standard timing than any of the other modes, only with whatever refresh rate the input was.
On my home theatre projector, previously, 240×3 and 480×2 did not work at all for any console, nor did GBI-ULL work in 240×2.
With the new firmware, 240×4 works on every console, and GBI-ULL works on the new 240×1 and 240×4 modes. I don’t know why ULL works on 240×4 but not 240×2, but I’m not complaining.
x5 seems to be unreliable both on my projector (which only sometimes syncs depending on what the previous mode was, and even then with random glitching and incorrect image shapes) and on the OSSC (which doesn’t seem to change output resolutions when you change them, after you change the x5 output resolution you need to leave x5 mode and come back in for it to take effect).
x4 is, with my projector at least, more compatible even than x2. Only downside is scanlines don’t work right in x4: they’re uneven. This may be due to the 960p -> 1080p scale. They might work fine on a 960p monitor.