It's not that many years ago when a 4" roller was better known as a rad roller, and although awkward presenting a tight fit they did allow the wall area behind the radiator to be painted. If a job is worth doing then do it.
I stopped taking rads off years ago, got fed up spending ages messing about with dodgy old valves that would leak once disturbed and packing glands failing. The thing is I'm a decorator not a plumber and so I'm not kitted out for these things happening, I tell my customers if they want them taking off for papering then get a plumber in, if the walls are to be painted I'll use a rad roller. Also check your public liabilities insurance regarding painters doing plumbing, you may not be covered if it goes wrong.
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