The number of owners doesn't matter as much as the car's maintenance history, which you already know. Some people equate MORE owners to LESS careful attention to maintenance. That's getting into statistical probability - I certainly don't believe it to be a universal truth. If the car has had 4 careful owners, then you're good. If it has had 4 owners, and 2 of them were rental companies or abused the car in some way, well then the answer is obvious.
In order to satisfy my own curiousity, I have removed "number of owners" as a deciding factor from my list, because there are (in my searching) plenty of cars with 5 or more owners, which seem to be well sorted (have not yet had any PPI's conducted). I might use it as a bargaining chip if the car I want has a "higher" number of owners, but that's about it. The car has to stand alone, you're not buying the previous owners.
Also, review the length of ownership for each occurance carefully. If it is in weeks or on the order of like 3 months, then can you really count them as an owner? Or were they a dealer who took title while selling?
This search becomes a deeply personal endeavor. I'm learning more about what were "must-have" criteria versus those which REALLY matter to me all the time.