So the multi switch setup as per below diagram:
I wanted to insert Shelly 1L under Switch 1 (it has the power) but I only found a diagram that would allow to connect 2 switches, not 3 or more. Is this at all possible without rewiring (every switch has 2 cables running).
I might be wrong, but I don't think it will be possible with only 2 wires between each switch.
My logic is that as a minimum, you need a cable running from the output of the Shelly from Switch 1 box all the way through to Switch 4 box, leaving you with only 1 cable to handle the switching. In the Shelly 1L, you have to run a wire from Sx back to SW1 or SW2, which would mean you need a cable going out and coming back - a minimum of 2 more cables, just for the switching.
I'd be happy to be corrected though, if someone else can figure it out with only 2 cables between each switch!
Could you do with only 1 switch (it could be any of them - I know it's not ideal...)? If you can do with only 1, the Shelly 1L can be placed in that box, and it would be possible to set it up.
You need to place your shelly after switch 4.
Alternatively you get momentary switches, the ones that turn on when you press and off when you release. You'd have to change the switches but you can leave the wires where they are. I did the same for a setup with 3 switches and the dimmer which worked, here's my idea for your situation:
The red wire is L so always power on this. The green wire only gets power the moment you press one of each four switches. Shelly 1L doesn't need a neutral wire (I didn't work with that yet so the wiring is not exactly correct in the shelly I drew but I bet you'll make it work) so problem solved 
