Expanding your trust list out to level 4 means that a lot of scammers will stay in your trust network which means that their alts will possible have positive trust. The scammers will also have the ability to make it easier to give people negative trust that probably shouldn't have negative trust (just look at how many people woodcollector and his alts have given negative trust to, and I think he is (or at least was) in level 3 or 4 default trust (which is the case with a lot of other scammers)