If zero rates with continuous compounding for 4 and 5 years are 4% and 5%...

When rates are continuously compounded, we can calculate the marginal rate for year 5 as (5*5% - 4*4%) = 9%. (Note that things would be different if the rates were not continuously compounded, and they were annually compounded. In that case we would need to do a calculation of (1.05^5 / 1.04^4) - 1)