I have P.T. installed on my PS4, but I am worried if I delete it. Since P.T. was removed from the PlayStation Store, if I uninstall it, will I be able to reinstall it at a later date?
Don't delete P.T.! If you uninstall it from your PS4, you will not be able to reinstall it normally.
Here is an article describing how, even though it seems like you would be able to redownload later, you cannot.
That said, it looks like there is a somewhat questionable method to redownload it if you do uninstall it, described here, but I highly recommend keeping it, just in case this doesn't work.