Mentioning possible PhD principal investigators in graduate application w/o being asked to do so?
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.