Unfortunately, people assume that if the “action level” is listed at 4 pCi/L, then a radon level of less than 4 pCi/L is safe. Scientists believe that any measurable amount of radon is unsafe long-term. For most people, their greatest exposure to radon will be in their homes; especially in rooms that are in contact with the ground (concrete slab homes), below grade such as basements, and rooms above crawlspaces.
Note: Please read the EPA protocols for radon measurements in homes. They vary widely depending on what type test was done, how long it was set for, etc. Basically after following all the protocols, a reading of 4.0 pCi/L or over should require mitigation. The rule of thumb is, the less radon, the better.
For more information: http://www.epa.gov/radon/pdfs/homes_protocols.pdf