For my Subwoofer amp, do I need specifically four 15v fuses or can i have different types that add up to 60v?

No you shouldn't. There is a reason why there are 4 fuse slots and not just 1 or 2. The terminals like dragonbreath said are soldered and that solder could melt if too much current goes into it so the load is distributed across 4 slots. On the other hand I don't think that the differnce from 15 to 20 amps is that much so it would porbably be ok but if the terminal melts don't blame me.