Does 4 hour nap mess up deep sleep?

So I've been coming home every day really tired and I'll take anywhere between a 2 and 4 hour nap. Like tonight I slept 4 hours, but I had to get up and do some homework and take a shower. So, if I fall back asleep later that night and sleep another 4 hours until morning (so 8 hours of sleep in all) is that alright? Or does it mess up my REM sleep or something by sleeping in 4 hour increments?