Here is a link to the official pamphlet from the EPA on levels and what is considered dangerous or not, and mitigation measures.
Statitistics show that for smokers radon exposure over 4 pCi/L roughly doubles lung cancer risk - but for never-smokers (meaning never a smoker and never lived for extended period with a smoker) the risk actually shows no statistical increase in cancer until the level reaches about 20, and at levels of 100 the cancer risk increase is 16% for non-smokers - so as you can see, the 4 level is pretty conservative. The EPA has established the "concern" level at 4 based on the doubling of cancer risk for smokers. (By comparison, the average home in the US has a level of about 1.3 - in UK about 20 is the average, for comparison.
Pay attention also that the level of 4 being a level of concern is for a lifetime exposure - if your basement has a higher levell but you do not spend most of your time there, then your average exposure would be much lower. A reading of 5 wold not normally be a cause for concern - the recommendation would be (assuming this was from a spot test) would be followup tests in the basement area and the upstairs living area away from the basement door, and to pro-rate the reading from those two tests based on percentage of time spent in each area - for instance if you spend 20% of your time in the basement at 5.1 and 80% upstairs at say 1.5 (or whatever your reading happens to be), then the composite average daily exposure would be 0.2 x 5.1 + 0.8 x 1.5 = 2.2 - only about half the "concern" level. In most cases a reading below 20 in the basement results in an average exposure of less than 4 overall, so unless someone has a bedroom in the basement or spend an incredible amount of time there playing video games or whatever, you probably have no cause for concern. Of course, how much it concerns you is a personal matter. In my case, having had extensive hazardous materials and waste handling training (including Radon and Asbestos Mitigation certification) I would have no concern below 10 even with a bedroom in that level exposure because usually more than half one's time would be outdoors, at work or school, upstairs, etc; and for casual exposure like a rec room or storage or workshop I would not be concerned below a level of 20 except with young children whose lungs are still developing.
Bottom line - I would recommend nothing if that was a basement reading rather than household average, or at most a 2-level check with a more accurate test method and then do the average exposure computation.
Should you decide to take active measures, they can run from about $300-500 for simple added ventilation to around $1000 for a vapor barrier and active extraction fan for a crawlspace to $1500-3000 for an active extraction system that removes air from under the basement slab through holes drilled in the slab and ejects the filtered air to the outside.
