I think you misunderstood the quote:
As for cancer risk, 1 million people flying 10 times a week will have 4 additional cases of cancer (using current models of radiation-cancer association). This is compared to the 600 cases of cancer they will get from the flight itself and to the 400,00 cases these people will have over their lifetime.
This means that for every 1 million people who fly 10 times a week, there will be 4 cases of cancer during their entire lifetime, not each week. This is compared to 600 cases they get from the radiation they are exposed to during flying and the 400,000 background lifetime risk (the average lifetime risk for cancer is ~40%).
Again, whether this 4 cases are worth it is a different question, but assuming the devices work as planned and are not delivering more radiation than they are supposed to, the additional cancer cases are pretty negligible (remember, those cases are for people that are flying a lot).