It's not good. besides affecting your mood and so forth the next day, it will create a sleep defecit. Contrary to popular opinion, a sleep deficit is not paid off simply by sleeping the number of hours you lost extra the next day. Sleep deficits take an exponentially greater amount of sleep to cover because it causes damage that has to be repaired.
I'm not explaining this in medical terms, there are articles about it that will use more precise language. But I am sure you get the gist. What you lose from the loss of four hours of sleep cannot be regained from 4 extra hours the next day.
Of course if it is just once, you probably won't notice the affect. but if it becomes habitual... one study I read found that people don't even realize that they are operating at a subpar level. The chronic affects of not sleeping enough accumulates slowly enough that 3 weeks later you don't realize you are running at 50% efficiency. You feel normal, but in comparison to the well-rested you 3 weeks ago, you are in poor mental shape.