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.
Finishing your PhD doesn't stop you from continuing in your field and creating additional publications. It is more likely people will look at your individual publications that at the contents of your thesis - but they will see that it took you five years.
If your department has funding to keep you after your defense, get the thesis wrapped up and keep publishing. It will look better.
There are two separate questions that could be asked here:
- Is it better or worse (or the same) to do a PhD in 5 years instead of 4?
- How does it look to others viewing the CV of a person who does a PhD in 5 years instead of 4?
The first question is a perfectly reasonable question, and we can all imagine the cost/benefit factors that might go into answering it (it depends on a lot of factors).
But since the actual question being asked is #2, I'll ignore all of that cost/benefit analysis and say: basically, they look almost identical to others. In my experience, when people are being evaluated for the purposes of hiring (or promotion or awards), almost always the chronological datum that is used is "years since completion of the PhD". In that case, the number of years it takes to complete a PhD is (axiomatically) irrelevant.
Even if there is some tiny difference for the very first round of hiring, the quality of the letters of recommendation is far more important than the historical data on the CV.