Sorry to have started this ball rolling and then not replied to the many good comments. I've been out for nearly a week.
Update on what I've done here:
1) While I was gone, I killed the breaker. Upon return, I unplugged everything from the bad circuit and flipped breaker back on.
2) I have two 3-light testers, a Greenlee GT-10 and a Woodhead 1750. Both provide correct readings on known good circuits.
3) Both testers show the first 3 outlets in the circuit to be good. All outlets downstream of the 3rd show Hot & Ground reversed.
I deduce from this that either a) outlet 4 is bad or b) wiring between outlets 3 and 4 is bad. Are there other possibilities?