Replying to 4.0 GPA scale vs. 5.0 GPA scale

For a project in one of my classes, the group I'm in is trying to change the 4.0 GPA scale we have at my high school to a 5.0 scale, where Honors and AP classes would be weighted. We have to present this to the principal and the school board, and therefore need a good argument that refutes any reasons for having a 4.0 scale.

So, what are some reasons we should have a 4.0 scale, or why do you think we should? Also, any input on why a 5.0 system would work would be great too.