I wonder if you're having the same problem I had with my 4200. After about a year in service, mine started squealing too. There was a slight leak at the shaft seal, and it destroyed the motor bearings. Sequence provided a loaner pump, and replaced the seals and the entire motor end of the pump (basically everything but the impeller and volute) under warranty, and they never even asked to see my purchase invoice. You should really give them a call if you haven't already.
Rogger's Dart should be a simple drop in replacement. I also have the Cimmaron pump that sequence sent me as a loaner (they let me keep it for a great price). It's basically a FW version of the dart, but they say it's fine for short-term use in saltwater.