There is a lot of fascinating info. about the pseudo science used by the EPA to develop US standards for acceptable radon levels, available online. Not, paranoid ramblings, but scientists who had a real knowledge of the risks involved, but were driven from conversation, as the truth is quite damaging to the multi-billion dollar a year mitigation industry. Radon is real, it causes lung cancer, and in long term exposures to high levels it will kill you. Is this reflected in the 4 pc/l level that seems to result in mass panic and immediate mitigation as a home is being vetted for sale? Hell no. First, relying on a single test is really bad science. Second, the "science" used to develop safe levels in the states is deeply flawed. Third, the levels used in the states work out to roughly 1/4 of the safe limits used in other countries. Personally, if I had a reading of over 15pc/l in a continually occupied space, NOT, the damn unfinished basement, I would retest several times to see if the level is reproducible, then address it. This bullshit of, "we found a 4.0 in your unfinished basement, your kids will die if you don't spend $2000 to mitigate, IMMEDIATELY" is getting old.
Bottom line? Radon is a legitimate hazard. Radon is also a grossly overblown issue in the states, and a cash cow for the mitigation industry, that has a nefarious interest in keeping the EPA singing the right song, and the cash flowing.