Mentioning possible PhD principal investigators in graduate application w/o being asked to do so? My title is pretty self-explanatory, but I was wondering how it looks to mention specific researchers to work with in graduate school via my personal statement/s. I'm applying to PhD programs in neurobiology (neurological disease/therapeutics research). 

I know the general consensus is "yes, mention researchers you would want to work with," but I'm worried that this may pigeonhole my application or limit it in certain ways. For example, if a new PI is not listed on the page but does research I like, and I don't list him, I lose that opportunity. Similarly, I'm worried that if I list, say 4 PIs to work with, and none of them care to have me work in their labs, I may get denied all together from the school's program (when I could have gotten accepted if I listed other PIs that may have wanted me). 

Also to note, at each graduate school I'm applying to, I have made sure that there are at least 4 labs I would want to work in throughout completing my PhD thesis.

This is a minor nuance in my application's personal statements, but I would appreciate any insight. Thank you very much!
I believe in this situation you have to decide on a proper ordinal scale for schools, faculty and the program. Since you mention that you have done the legwork and identified 4 labs, it would make sense at this point to possibly write to them and ask if they might have openings in your projects of interest.

Additionally, I don't believe faculty will disregard you for listing their colleagues, unless they have a personal feud. Younger faculty especially, will consider students who might have listed more senior faculty, if they themselves have not made it clear that they are looking for students. i.e, if a new assistant professor has not set up a group website, but sees a student has applied to a senior colleague in the same field, he will most definitely consider you favorably.

The point to remember is that you are looking out to do research you find interesting and in a field where you feel you will be able to make a meaningful difference. It is not a good idea to be vague in hopes of acceptance if you are not sure of working with the PI in question anyway.