I happen to have the 4500 and my in-laws have the 4520 which I helped set up for them. The 4520 is obviously a new model and has a nice upgrade that I like a lot. - the automatic paper output support. This is the flat plastic bar that holds the printouts. If the paper output support is not extended in the 4520 prior, no worries, because when you send a print job, it automatically extends out to keep print-outs from falling. My 4500 is inside a cabinet whose door is always open but we keep the paper support folded and inevitable the printouts always end up all over the floor if we don't remember to extend the bar. HP engineers took this problem away in the new model!