Is a large change in radon test results normal?
You can get the test re-done, they generally aren't very expensive, but avoid the home testing kits, they aren't as reliable, and any buyer will probably have the test done themselves anyway professionally. It is a significant jump, it definitely crosses the threshold of "safe" and "needs correction". Any smart buyer will want you to correct a value this high, too. A test result >= 4 pCi/L warrants some kind of radon abatement according the the EPA.