Please note: The Crydom relays that we sell are designed for AC circuits only -- they will not turn off if you use them with DC. Crydom does make a line of DC relays that will fit into the RC-4. Here's one example:
http://www.crydom.com/userResources/productFamilies/141/Crydom_CMX60D20.pdf
For you geeks that want to know why... SSRs typically use a triac (http://en.wikipedia.org/wiki/TRIAC) to do the switching. Once turned on a triac will not turn off until external power is removed. In an AC circuit this happens 120 times each second (in the US, 100 in Europe and other 50 Hz areas) at the zero-cross point. In a DC circuit there is no zero-cross until you pull the plug!