Rock N Play Sleeper

So...I'm totally freaking out. I'm a first time mom, my son is 4 months old and he's been sleeping in his Rock n Play for several months now. However, I just read a post that said it isn't safe for babies to sleep in a Rock n Play. Have other moms used this for nighttime sleep? I'm working on transitioning him into his crib but I feel horrible and am super paranoid now. :(