You mentioned weight and costs and those are two huge factors for motion imagery, no so much for stills.
The S at 4k is going to be difficult with wide angles, as at 4k th epixel is one to one, so the sensor crops.
For some production if you want to go wide and use PL's that's great, but if weight is an issue an S with a few lenses, a recorder for 4k and the weight and costs go high.
The best option would be a true dedicated 4k video cam like a C500 or Sony has a new $10,000 FE mount video cam, but neither are build for stills.
I would think your best options would be.
1. Keep your Leica and buy a A7S with a 4k recorder and an adapter for your M lenses. The only downside is with a 4k recorder and two different bodies is multiple battery chargers, a slightly larger rig with the A7s and no autofocus, though with m lenses you'll have a small kit.
The main downside with the A7s is reading a full Frame 35mm frame in video will distort (jello very easily) so pans, car mounts, even walking with a stabilizer can make for some very funky looks.
.2. A Canon 1dc that shoots a 18mpx still and 4k video, though it is a larger camera than the Leica and it's not cheap, though does both stills and motion imagery but cost and weight is probably a deal killer for you.
3. I'd look at the Samsung NX1. it's 4k 40MbitsS (which is kind of the standard for small cameras), though shoots a 4:2:0 8 bit video file which is kind of week and you're exposures are going to have to be dead on.
Still the NX1 looks interesting and apsc (super 35) is the film makers format of choice.
They make a 16 to 50 and 50 to 150 both constant 2.8 lenses with satabilization which will cover about anything anyone could shoot (approx 24mm to 230mm in full frame 35mm still terms).
Also you're going to need to add external ND's (or a fader) but that holds true on all the dslr type cameras I've mentioned.
My only caution would be on the NX1 is to wait and see how it works in real world.
4. Keep your Leica 240, learn Da Vinci Resolve, improve your grading and editing skills and then look at more robust equipment.
5. Rent first from borrow lens or any of the discount rental suppliers. You can learn more from that than reading anything on the web.
Keep in mind the main reason to shoot 4k today is not 4k delivery. Right now, few monitors, even computers will show a 4k file, but shooting 4k or more cuts moire, alaising, and allows a little more cropping than 2k, but once again you shoot 4k for the look, but I wouldn't kill myself trying to deliver 4k for personal use.
IMO
BC