Firstly, the numbers you refer to are on the TRV ( look it up) and related to how warm the room is. Say that 4 represents 20 degrees, then the valve would be open until the room gets to the 20 degrees, at which point the valve will close and the radiator will start to cool down.
So set the number to how warm you want the room, there will be some trail and error. Trv's are good, the radiator only demands water when the room is colder than you would like