i say move up from 4psi in 1 psi increments. wear GLOVES and EYE PROTECTION!
if it were me, i'd want the actuator switch on a foot petal so i could use both hands to move/manage bottles and caps. just something to think about for v1.1, you could always just splice a quick connect jack into the existing button, then you'd have the best of both worlds.
gotta say, awesome freaking build... and if you don't post videos we WILL find you and make you drink Natty Ice as punishment.