As with most problems, there are different ways to skin this cat and each comes with some level of compromise.
In general 1-4 are correct. The shorter your focal length, the longer exposure you can get away with before the stars start to trail. The longer the exposure, the lower you can go on the ISO, though at F4 you'll need to be at least at 3200 I'd think, more likely 6400.
So, if you go with either your 40 or 50, you're going to have to use a shorter exposure time (I'd guess you can get away with about 10 seconds on full frame, less on a crop body). That shorter time will require a higher ISO, so more noise.
As for stacking, I've seen some really awesome stacked shots, and some equally awesome single shots. Depends on your purpose really.
If I were you, I'd start simply. Use your widest lens, ISO 6400 If your body can do it, largest aperture. Once you've got a few shots there, start experimenting with different time/ISO combos (I wouldn't stop down past F4). Once you have that down, slap on on of the other lenses, open it up, halve the exposure time and maybe the ISO, and repeat. You'll find your sweet spot after a few tries.
A few tips:
Find a good foreground BEFORE it gets dark. Most places you'll want to do this are very dark indeed.
Get a headlamp or cap light. You'll use it a ton.
Turn off autofocus. Use live view and magnify on the brightest star you can find, focus on it manually, then turn off live view to shoot.
Get a cheap remote release. Wired is fine, unless you want to be in the shot. It can be a pain to rely on a 2 second timer.
If you want to illuminate (light paint) any foreground, get a variable power flashlight. Consider bouncing it off your hand to avoid hotspots.
Hope these help.