I just used the KiCad defaults in the PBC editor (File / Board Setup, Board Stackup, Physical Stackup : Copper Layers = 4). I've done several 4-layer boards like this, sending the Gerbers to JLCPCB and they come back fine. When setting up the JLCPCB job you have to assign each layer to a particular Gerber file, but that's pretty obvious.
I'm not doing anything fancy with controlled impedance or the like. I could give you the thickness numbers that KiCad uses, but they are just the defaults, I haven't changed a thing.