I have a wall-mounted gas boiler and four radiators around the house in different rooms and locations, and the thermostat is in the living room. I wonder should I close all radiators except the one in the living room during the day when I spend all my time in the living room, or just use the one in the bedroom during the night, or it is more efficient to use all radiators. I ask this because when I use only one radiator, the boiler is almost always on to keep the temperature in the desired range, and I wonder isn't it more efficient to use all the heat in the hot water before it goes back to the boiler?
If you turn off 3 of the 4 radiators, then 3 of your 4 rooms will start to get chilly. It will likely take far longer than you'd like for you to warm up the bedroom after it's spent all day getting cold.
Of course, you could try it to find out. Your best bet would probably be to start on Feb 1 or whatever the first day of your next heating utility billing cycle is. Run your plan of turning off (or down) all the radiators in the rooms you're not currently using every day for the February billing month and see if A) you like the temperature in your house, and B) if it actually saves you any money.
You'll find that at night, when you've only got the bedroom radiator turned on, that the heat will run all night. You're only providing heat in the bedroom, so it will have to flow by natural convection (warm air moving through the house) from the bedroom to the thermostat in the living room. Depending on your house layout, your bedroom will probably end up at 85°F † for the living room thermostat to be registering 68°F † and kick off the heat. If you're comfortable with that, then run with it, but I'll bet you and/or other house occupants won't enjoy it.
Of course, you'll have to adjust your spend for the number of heating days in Jan vs Feb, since temperatures can be different and the amount of heat required to keep the interior will be different based on the exterior temperature. Just do some internet searches for "heating days" and you'll find the numbers for Jan & Feb 2022 for your town and you'll find formulas for how to adjust your heating bill to accommodate the difference.
I'll bet that you won't find that it's as big a cost savings as you're imagining, and that you won't like being in the house where 3/4 of it is much colder than you're used to. On the other hand, if you have a very well sealed and insulated house, you may just find that it's perfectly acceptable and that the few (units of currency) you save make it worthwhile for you.
† Made up internet numbers for the sake of argument and example. Your actual numbers may vary, or may be indicated in °C instead of °F.