Pat's right. If you got a set of four Polks, you should complete the set by adding either a matching center speaker, or a fifth Polk bookshelf speaker (this is the way to go if your TV is wallmounted or otherwise positioned in such a way that you can line up three identical speakers up front). Since you already use a subwoofer, you won't really need floorstanding speakers, unless you prefer to listen to two-channel music sources with the speakers at full range (and the sub switched off).

Anyway, get to know the receiver and its many functions first. Then, decide if you want to start doing speaker upgrades. Keep in mind that once you upgrade the front speakers, then you'll want to eventually match the voicing all the way around for ideal surround performance. So, it's not as simple as changing one set of speakers.