geekIT:
Mike, how did you come to get 3 primary shots? Re a 4th shot for you, AFAIK they're all the same Pfizer dose anyway. I'd happily rush off and get a 4th if it was offered. I had no reactions to 1 and 2. After getting the third the jab site was slightly sore for a couple of days. I'm 81.
In December people who are seriously immune compromised became eligible for a 3rd Primary dose. I received mine just before Christmas. This had to be done by a prescription by my GP under special authority and I had to sign that I understood the risk and agree to have the dose. I was advised that this not the same as the booster.
I come eligible for the booster in March.