Hi Jens,
Your idea looks cool!
I have several notices on this, which may help you to avoid some problems.
1. Slim camera cables
- Actually, there are no such a cables on market. They exist for another cameras (as a rule for the FPV drones), like this for RunCam or this one for Mobius, but incompatible with RPi cameras.
- Also, if you will use a classic camera ribbon cables, but longer (50 cm, 1 m etc), you can face a problem with capturing electromagnetic noise on that cable. Camera will be detected, but will not work. You need to do a shield to defend it.
- You can consider using HDMI cable to extend camera cable length. There are several adapters on market, like this one. With this kit you can do a VERY long cables (like 5 meters).
2. Combining 4 cameras
It is not a problem to use a couple of StereoPis and get 4 videos. But stitching is a challenge. For real-time stitching you'll need to find good algorithm and create a code for the real-time stitching. I don't know any ready-to-use solutions for this (but may be my google skills are not so good

).
I'd prefer to do a simple MVP to test this concept before spending your money on StereoPi, cameras and cable extenders. For example, you can take several PiZeros with the cameras, and stream a video from them to a "main" device like RPi4 (by Ethernrt or WiFi). If this will work - you can try to implement more advanced solution, using StereoPi. IMHO stitching of 4 videos is the key potential problem here (RPi performance). May be NVidia Jetson as a big brother for stitching?..
Offtop: before creating the StereoPi, we've tested a concept of stereoscopic video with by creating this simple add-on to the RPi camera, 3Dberry:
http://3dberry.org/sites/default/files/ ... _light.jpg And only after that we decided to spend our time and money for the StereoPi development, as this simple device shows us, that this approach is working.
Eugene a.k.a. Realizator