My employer contributes 4% of my gross income directly to my pension plan. Am I am only able to save (18% - 4%)=14% of my gross income in RRSP's without overcontributing?

Your RRSP deduction limit is calculated by multiplying 18 percent of your income and subtracting any pension adjustment amount that you might have. The RRSP deduction limit is calculated when your income tax return is assessed by the Canada Revenue Agency.  As long as you do not contribute more than your RRSP deduction limit which is identified on your Notice of Assessment, you will not need to worry about overcontributing.

https://turbotax.community.intuit.ca/replies/2647951