If total is 4 litres it means that it also accomodates heater and expansion bottle.
To flush out, remove lower radiator hose and use garden pipe into a heater connection so it flows from the head to the block to waterpump and out the radiator.
Quite simple,
Then fill with clean ro or distilled water and drain once or twice, make sure no leaks exist, install new t stat and then fill up 1.8 litre coolant concentrate, start engine and refill rest with clean ro/distilled water till no more water is taken in. Cap the system and leave engine running.
With remaining 200 ml concentrate, add 200 mil water and fill expansion tank slightly above max and wait till cooling fan starts. Then shut down engine and leave the car alone for an hour or so, if everything is airtight and good some coolant will have been sucked out of the expansion tank.
Its a good idea to clean the expansion tank with the acid too so you can actually see through it, while it at make sure the hose connections are tight and correct at the cap so it doesnt suck air from a loose or punctured hose.