I have a 4 cylinder block that just has 1 bad bore. Can you just fix that one or do I need to have the whole block done?

Almost certainly it will cost less to refinish all 4 bores than it would to mask the “good” ones and just fix the bad one. Of course, you should be asking yourself if the good ones are really still within specs for optimum performance, or if the whole block needs to be reconditioned anyway.