Was wondering if anybody knew if I can use the boxes left by another customer for new service, or should I just return them, cause I have four boxes and a router left here when I moved into my new place
Interesting question. All Verizon boxes are rented. So you won't save money by keeping them. As far as I know, you would have to have those boxes activated on your new account to continue using them. Also, your new account would have to have the same number (four) and type (SD/HD STB/DVR) of boxes in order to use all of them. If you like the models that are left, Verizon might be able to allow you to keep them. All you are really doing is saving Verizon some shipping or installation costs. The previous owner might be in some trouble for not returning them, but I don't think that should be your concern.
Some STB/DVR models are much better than others. I would say hold onto them, check their model #'s, and if you like them better than the ones you are offered when you get your service installed, see if you can keep them. Then return the ones you don't want.