There's no enforcement on the methods you employ as long as it follows the rest of the rules and looks like a gameboy game. One thing worth being careful of with scaling vector art is it leaves traces of anti aliasing which will put you way over the 4 color limit. Also, it shouldn't just "look like" 160x144 - the gameplay area needs to be 160x144. Obviously you can scale it up - x3 or x4 is a good spot. But you can't make the resolution 640x576 just because it's 160x144 multiplied by 4 if that makes sense.