A new PC4 is $90
A used PC4 will probably run in the neighborhood of $55-$70 depending upon age.
A brand new Reefkeeper Lite is $100, only $10 more than a brand new PC4. The new RKL includes the PC4, as well as a new head unit and new temp probe. If you decide you only really want the PC4, you can probably sell the brand new head unit and new temp probe for $25-$35 (or even keep them for another setup. They wind up to be cheap enough.).
My point is, you might want to consider picking up a new RKL to get the component you need.