The second part of a continuous 8hr sleep (or a 2x4hr sleep that’s only broken to pee or a random waking) has a greater percentage of REM sleep as compared to the first which has more deep sleep. So I suspect if one were to try 2x4hrs with a significant gap between the two they’d eventually end up short on REM sleep and suffer the consequences of that, which would be similar to short sleeping every night. Fatigue, memory loss, greater risk of dimentia, poor body composition…etc. I don’t know if there’s been studies done on this exact idea though.
Given the body’s natural circadian rhythm, I think overall sleep would suffer in at least one of those stints too…because your body’s not at the right temperature and it’s too near a daylight hour and you wouldn’t have developed enough adenosine to feel sleepy…etc