I am talking internally with the customer support team about what happened to this customer's traffic. It is definitively not the case that doing "four requests per minute" gets you rate limited.
I'm a free plan user because my application is not monetized and obviously would switch to paid plan once it starts generating money. CF free plan has been indispensable for many such free projects.
UptimeRobot(Free) doesn't show anything fishy, except for CF error 521 occasionally. Again UptimeRobot requires paid version to check for HTTP status message, guess I'll have to do it or setup a self hosted(recommendations?) solution to monitor HTTP status of CF Free application.
If CF does perform rate limiting for such low rpm, I would prefer to be intimated as even third rate shared hosting providers do it and I hope CF wouldn't put their reputation on the line for something like this.