Flashing a Sonoff 4ch is the same procedure as flashing a Sonoff Basic.

How to flash Tasmota firmware to Sonoff basic

Flash button

Set Sonoff 4Ch R2 in Flash mode:

(1)UNPLUG your FT232RL from your computer.
(2)Press and hold the push button (100) on the Sonoff board.
(3)while holding the push button insert the FT232RL in your computer.
(4)After 3 seconds, release the push button.

Sonoff 4ch R2 Template:

{"NAME":"Sonoff 4CH","GPIO":[17,255,255,255,23,22,18,19,21,56,20,24,0],"FLAG":0,"BASE":7}