First, the validity of your PR card and your PR 'clock' are two different things. On what date did you land in Canada? You say about 4 years ago, so let's say you landed in October 2010. Your got your PR card in November 2010 with an expiry date of November 2015. You stayed in Canada for a few months, let's say until March 2011, and then you went back to your home country and have not been back to Canada since.
Your PR 'clock' started in October 2010 - when you landed.
You will be a PR until you either give it up, or until CIC takes steps to revoke your PR status.
Basically, when you get to the border, it will be a gamble, and no one can say whether or not it will be successful. It will come down to whether the CBSA officer files the paperwork to start proceedings to remove your PR status when you cross. That may or may not happen, and no way to guess beforehand whether or not it will.
Best case scenario, CBSA asks minimal questions and you are let into Canada with no real fuss. From this point, you stay put in Canada until you have the 730 days, then you apply for renewal of your PR card. You won't be able to travel outside of Canada during this time, but you should be just fine to have your PR renewed.
Worst case scenario, CBSA puts together that you will not be able to meet your residency requirement, and starts the process to revoke your PR. However, they will still let you into Canada as you are technically still a PR until the status has been formally revoked. You'll wait in Canada and you can look for work etc as you are still a valid PR. You'll receive notice from CIC that your PR has been revoked because you have not maintained the residency requirement. At this point, you can file an appeal on H&C grounds given that your wife and children reside in Canada. The decision on the appeal could go either way, but you can remain in Canada while the appeal is processing. I would recommend reading case notes on CANLII to see cases where appeals have and have not been successful. I would also hire a lawyer to help you at this point.
I would not bother with the lawyer right now - the lawyer cannot predict nor influence what will happen at the border when you get to Canada. It really will just depend on the officer, what questions you are asked, and whether the officer decides to start paperwork or not. No one can predict whether or not that will happen - as I said, it will be a gamble.
You losing your PR would not affect your wife and children's PR statuses as they have met their residency requirements. If your PR is revoked, then your wife can lodge another sponsorship application at that time (I think).
Hopefully that helps...
