I don't think it's a cause for concern but I would look at seeing if there's a way to feel refreshed with a longer sleep.
It might just be that your natural sleep cycle is almost exactly two hours long (https://en.wikipedia.org/wiki/Sleep_cycle). Therefore at four hours you have completed a cycle, whereas at seven hours you are halfway through one. Perhaps try eight hours and see what happens?
Out of curiosity do you use an alarm to wake yourself up, and did you before? I'd be quite surprised if someone could sustain four hours sleep a night without an alarm. I've often heard that if you need an alarm to wake up then you're not sleeping sufficiently.
It might just be that your natural sleep cycle is almost exactly two hours long (https://en.wikipedia.org/wiki/Sleep_cycle). Therefore at four hours you have completed a cycle, whereas at seven hours you are halfway through one. Perhaps try eight hours and see what happens?
Out of curiosity do you use an alarm to wake yourself up, and did you before? I'd be quite surprised if someone could sustain four hours sleep a night without an alarm. I've often heard that if you need an alarm to wake up then you're not sleeping sufficiently.