Doubling every 4 days is the worst I have seen. Doubling every three days is off.
Doubling every four days where? Not in the US. I just looked at the CDC web page, and from 3/1 to 3/11 the average daily increase in new cases fluctuated between 50 and 113, with the 50 number being the last in the series. There was no spiraling upward trend. The CDC reports about 4,200 cases, with 879 new cases between 3/1 and 3/11. That would be a 26% increase over 11 days. If a few more late-reporting cases come in for those dates, that might raise it to 30-40% over 11 days. That’s far short of 100% every four days.
What are you talking about?
Real doubling time (2 days or 3 days or 4 days) will not be known until testing rate catches up with infection rate. Until then, the rate of new cases will simply be a reflection of increased testing rate. In other words, more people found to be infected because more people were tested.