Tim it would be interesting to see how electrical resistance a 4-40 has. I have no way to check it.
If you can measure current and voltage simultaneously, you can measure the electrical resistance between any two points on your switch. You can even approximate it with your finger.
Specifically, you can get the thing running at the design current (i.e., run it on the ground with a cut-down prop), then measure the voltage from the screw head to the copper. You probably want to get the screw head to top layer as one measurement (there'll be some voltage drop), and a separate measurement from the screw head to the bottom layer.
Alternately, with that same cut-down prop, let it run for about 30 seconds or so with your fingertip on the screw heat. If it gets hot, the resistance is too high. If you're like me then you've done that test enough that you want to use the
back of a finger, because after burning yourself enough times your fingers get insensitive to heat...
Aluminium screw is also practical, steel screws are also an option (but they rust).
Any dissimilar metal contact at points like these will tend to cause accelerated corrosion. If there's any movement from vibration, you'll add fretting corrosion to the mix. It's best to use a screw of the same alloy as the bar, and to use something reasonably conductive (like brass) and something that has corrosion properties that make it a good switch material (like brass). Brass with a heavy gold plating would be better yet -- let me know when you find someone who will do that for cheap in small quantities.