They do mitigate the radon, which is what they’re supposed to do so the answer to “do they work?” is yes. You can get the readings from them to see how much and how quickly the radon is being mitigated. That said, although 4.0 is the maximum allowable amount set by the EPA, you should be aiming for a lot lower.

So is it reasonable to expect a mitigation system could get my reading from 4.0 down to 2.0 or less?

Yes. Some systems can reduce it almost entirely. The keyword here is almost - don’t expect it to reduce it 100%.

I have a radon mitigation system and it does work. I've had a system for over 20 years.

Yes, they do work... Properly installed radon mitigation system will bring radon levels to or below 2.0.

I would suspect it depends on the system type. Mine is tied into the drain tile around the foundation. Level looks pretty good to me.

Typical level for my area is 6 and there are 1000s of homes that don’t have radon mitigation. It doesn’t make it right to not have one but your level wouldn’t make me run if It’s the right house.

We had a radon mitigation system in our last house and it did work to bring the radon levels to almost 0. I do not know the original measurement, as the system was put in by owners that were several times removed from us.