How does it look if I finish my PhD in five years instead of four?
The quality of your thesis overshadows the length/duration of your PhD. 

Having said that, it depends on your goals. If you clearly think (and your adviser as well) that another year would make a big impact on the quality of your thesis and, consequently, on your CV, and is not too risky to pursue extra work (i.e., the questions you want to tackle are not too ambitious and you have a good possibility of enhancing your CV) then go for it. Otherwise, I am not sure you would want to stay at PhD while you could be looking for your next (PostDoc or industry) jobs.