I didn't know this. It makes sense, and it's also very much how things work for me. I usually wake up at those times, not refreshed, but also not very sleepy. If I wake up at like 1 hour only, I'm bad, or 2 hours, but it makes sense because I've noticed some times at which I can wake up.
However, for me it's not 4.5h but 4h most of the time. Exactly. Sometimes I go to sleep at like 11:45 and I wake up at exactly 3:45, not feeling refreshed, but very not-sleepy, and I can't sleep until a few hours later. If I go to sleep 3 hours later, for example, my body doesn't interpret it as 2x4h but as a complete cycle. Strange bodies!