FantechRn2 Radon Fan - 4.5" Duct - 123 cfm

Q: Looking at "b 4.5". current one is 4", to 3" pipes. other reviews/questions + product title indicate this is 4". is it 4" or 4.5"?

Always think pipe dimensions (the ID of the pipe) when dealing with pipe. The Rn2's top and bottom connections are 4.5" OD nominal which translates to 4" pipe. You use a 4x3 Flex Coupling with the Rn2 and 3" PVC pipe. The PVC vent pipe is usually 3" which translates to 3.5" OD x 3" ID Schedule 40 PVC.

Q: how many square feet of basement with a sump pump will this service

Basing fan selection on square footage of a basement is not exact since air flows will vary based on many factors. But in most cases the RN2 should be fine up to about 2500 sq/ft. This is a 4" fan so using 4" pipe will give the best results.

Q: Can this fan be on a switch and turned off when we are away?

As a radon professional, most of the fan failures I've seen in 20 years are when fans have been off for a period. Soil moisture coming out of the pipe and condensing on the metal parts (bearings) can ruin them. They seem to be good for at least 15 years running continuously when properly installed. If you have to shut it off, remove and store the fan and replace the space left by the fan with solid pipe so you don't have any other moisture issues. Shut off switches located near these fans is an NEC safety code requirement. Radon levels go right back up a few hours after turning a fan off and the converse is true.