PL 4 is not a superhero, it's just a semi-competent street fighter. Cops are PL 3, soldiers are PL 5. The book suggests PL 6 for low-power heroes, and I recommend going with that.

I don't think if making characters as superpowered later is a good thing to do - they will only be normals for one session at most, after all. It's much better if they already make their superhero sheets in advance, together and under DM supervision.