Cutting a 4" hole in a piece of .125 aluminum sheet

I'll suggest a little cruder approach. Maybe Dykem the plate and set the tubing on top. Scribe the perimeter on the plate. Then use a hole saw with repetitive cuts to just short of the line. You'll want to have the plate clamped down on the drill press/mill. Then use a drum sander in a DP or mill to sand to the scribed line. You could check the fit with the tubing and hit the high spots. You should be able to get within 1/16" of your line, a little gap will get filled with the weld.

Bruce
 
I recently made a welding positioner and had to make a control box for it. The box needed a cutout for a fan, I used a boring bar and a piece of MDF clamped in the vise, then clamped # 20 gauge sheet metal to the MDF. This achieved perfect accuracy for the hole diameter
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