I should have been more clear about the z-height. The distance from the table to the bottom of the bit, which is inserted fully, is about 2.5″. I have a 1″ block under the aluminum and 1/4″ thick material. That leaves me another 1.25″ to come up, but then I have no room to clear clamps, etc., because I will be at my maximum z-height. The 4″ is from the table to the bottom of the router body itself, when at maximum height.
I can certainly slow the speeds and try again, but I would like to make the machine as robust as possible so that I can cut some aluminum plate fairly routinely. I assume that I would be better off with a single flute bit?