How to cut many shallow grooves on the side of a square aluminium tube?
I believe this can be done in a home-shop environment using a table saw with a carbide tipped blade. I cut 25 mm square aluminum tube often enough without harming the saw. The swarf is a bit of a mess, as one might expect.

The challenge here is to properly index the cuts. As you require 4 mm slots, it means two passes for each slot, along with the need to index for each individual slot.

I think the best approach would be to construct a sled with the required angle and create a stop-block that is secured to the angled portion of the sled with indexed holes into which you'd thread a bolt or screw for more precision.

The requirement for two passes would be managed by a shim on the stop block. One pass for the main cut, insert the shim and make a second pass for the correct width, then relocate the stop block

From Lumberjocks.com:


The photo represents a simple, flexible saw sled for angle cuts. The indexing stop block could be attached to a longer piece of wood on the right side in the image. Alternatively, one could make the right block indexed to the sled or to the hold-down clamp.

This isn't necessarily the only answer, and is expected to be used as a starting point for expanding one's thought process to reach the goal.