Some people have "delayed sleep phase syndrome," where your natural rhythms are not necessarily in tune with the general population. I can relate! I kept exactly the same sleep schedule as you, from 4am to noon, through most of my own school years. If that is your natural sleep period then it's entirely workable as long as you're able to schedule your classes around it. I typically get a "second wind" late at night so I got my best studying done during those quiet hours after midnight, when there were fewer distractions.

However, if you're obliged to take early morning classes or this isn't your body's ideal time to sleep, then you'll need to make some adjustments. Experiment and find out what works... you might even find that sleeping into two separate 3-4 hour periods is more convenient than one 6-8 hour stint (think of the time in between as a really long WBTB!) Changing your sleep pattern is not easy if you're going against the pattern you're programmed with, but I'm convinced it is more important to get enough sleep than to worry about precisely when it happens. If you find yourself consistently lying awake during particular hours, then maybe those aren't your optimal sleep times and you should dedicate them to studying instead.

It's definitely possible to make adjustments if you need to. I've kept 9-5 jobs before, although I hated it. I even spent several semesters attending 8am classes, although I found my motivation for those subjects decreasing more precipitously than they might have. These days I'm able to maintain a later schedule and sleep from around 1:30 to 9:30am comfortably enough, though whenever I find myself in less structured time, the old 4am to noon pattern tends to reassert itself. You'll probably find that there are parts of the day or night that you never ever want to be awake for, and others when you find it almost impossible to fall asleep, so as you re-organize your schedule, try to take those into account.