Shouldn't be a problem.

Your fine. The 4" recommendation is to cover every scenario, such as people who put there rigs on long nap fiber rugs on the floor or throw rugs of weird build etc..... I would def suggest putting some sort of filter on it though, if it doesn't come with one, your PSU is the most important thing in the whole case and the last thing it needs is a bunch of dust stuck in it. I like this type, https://modmymods.com/phobya-fan-filter-120mm-black.html   because it screws a backing on to the fan and the cover and filter snap on that backing so when you have to clean it it is tooless and easy. They have magnetic filters that stick to your case, if its metal by the fans, like this http://www.newegg.com/Product/Product.aspx?Item=9SIA85V40T5813&cm_re=120mm_fan_filters-_-9SIA85V40T5813-_-Product  , but I wont use them myself because I don't want to put any magnetic fields near my components. Yes, I know its probably not going to hurt anything and its a very weak field but my paranoia wont let me take any chances on my main rig.