Your boiler should be able to heat just one rad, once the heating circuit is up to temp it should turn of the burner and just run the pump. If this is not happening you need to contact a heating engineer and sort the issue. Hacking a fix is not the way to go.
It is possible. The boiler puts out 4kw , heats the heating loop , once heating loop is up to temp the burner shuts down and pump keeps running. The 2kw rad dissipates heat and cools the loop, once loop cools the burner should fire up again. As long as there is a call for heat from a thermostat or smart valve, the boiler should just keep heating loop up to temp. Again if this is not happening you need to look at why.
Not a terribly efficient approach but I've seen a number of articles highlighting that there are boiler systems where going round and turning off TRV's in rooms you aren't using may be a bad thing for this very reason.
Unfortunately though, I'm not sure any of the "smart" heating systems support that kind of mode.
I will leave you to your opinion , but i will say that is how it works, and i have installed 1000's of heating systems in my time, and my home system can heat 1 rad with a 24kw boiler with no issues., my boiler will cycle the burner off and on, this is normal for a boiler once circuit is up to temp.
I have installed intergas boilers last one was about 1 year ago, can not remember the model but it was a hre boiler, it functions as i describe. Just looked at pdf for similar model, could not find a 26/24, and I see no reason why it can not heat a single rad.
you dont want to have a installtion that goes on or off, then it lose his rendement. radiator water need to come cool back to do codensating, so radiators are installed with a debiet, means less flow and intergas don't notice a quick diffrence in temp on the water retour hose
Boilers are designed to turn off and on when heating loop is upto temp, it will not affect efficiency (rendement) if you want to have efficient condensation set the boiler for 55 - 65 degrees C. If water is returning hot the boiler burner will shut down and condensation is moot at that time, as there is no condensation as burner is not burning.