CPP Contributions beyond 4 years

My employer set me up to contribute to CPP rather than SS for my first 4 (5?) years, living and working in the US. Just informed that for the last 3 years, neither CPP or SS was deducted from my pay. Payroll didn't begin SS deductions following CPP expiration ... and I didn't notice. Employer suggests I contact CCRA to see if I can extend CPP contributions and catch-up these missing years. 2011 will be the first year I paid SS (caught up before year-end) so I'm now "in" for 4-quarters of SS.

- Catching up CPP will be less expensive than SS, but what would be best for me at retirement time?

- Is there any likelihood RevCan will grant me (a certificate?) to retroactively contribute to CPP beyond the original 4 (5?) years?

follow up with me at forums.serbinski.com, this is an interesting question.
When you do, ad dinfo about how you are paid/taxed.