Joao,
I am not sure if I understand correctly.. you have 4 master pages without using the SplittApp control (as in a Master Detail page) ?
anyways, here is the walkt hrough documentation - SAPUI5 Explored
please look at step 31 -- routing and navigation. I hope this helps your scenario.
I am still interested in seeing how the 4 master page scenario without a splittapp control would look like. One thing i can think of is to have a page with tiles... and specific tile(s) would navigate to a master page (up to your 4 different ones).
hope this helps