Referring to your link H is the thickness of the dielectric between two copper layers. For a four layer board I guess it depends: if having an higher H can be good for you you can leave a copper layer "unpopulated" as in "with no copper", and make the plane in layer 3 or 4.
This calculator assumes that the bigger trace is a low impedance plane, ground, vcc or whatever, and that it's bigger than your microstrip, i.e. wider and longer. The last assumption is needed to neglect "edge effects" (is it correct?).
About your last sentence, if you make a microstrip on TL and one on BL for them to have the same characteristic resistance, apart w and t, you will need to provide a plane at the same H for each one. In a 4L board that can be achieved only with two planes in the intermediate layers, for a 5L board maybe one plane in L3 can do the job.
Keep in mind that you can bury a ustrip in an internal layer, here's another random calcolator, maybe that can suit your needs better.