Virgin Mobile USA = CDMA + LTE via Sprint
Virgin Mobile Canada = HSPA + LTE via Bell Canada
depending on what type of Virgin Mobile USA Plan you are on, you may be able to roam in Canada on one of the networks given you have roaming option available on your plan.
Also I don't understand by what you mean "Can't use iPhone in Canada"
That sounds like you've got your answer. Prior to the iPhone 5, CDMA and GSM phones were different models. I don't believe the CDMA iPhone 4 or 4s had GSM radios(I could be wrong on the 4s). It's may be POSSIBLE to roam in Canada or Mexico, but your phone doesn't have the capability.
There you go, that is your answer. Its not that the phone is not capable [iPhone 4 was not but 4S + are] but your carrier does not have roaming agreements with international carriers.