If the 1-year forward rates for years 1,2,3 and 4 are 2%, 3%, 4% and...

The zero coupon spot rate for 4 years can be calculated as = (1.02*1.03*1.04*1.05)^(1/4) - 1 = 3.49%, which is the correct answer. (3.50% is just the mathematical average of the rates for the four years and is not correct, even though close.)