Double Taxation?

My wife and I will be moving back to Canada in June after spending four years in China. We are so looking forward to returning. Canadian summers, winters etc, you just can't beat it!
Our employer has paid tax on our income while we have lived overseas. Will we have have to pay tax on our income in Canada as well for the past four years?

It's not clear what your tax situation is from that comment. If you cut residential ties to Canada and moved your tax home to China, then you are not liable to Canadian taxes, although you needed to tell the CRA that you left on your final T1 return for the year that you left.