Lynn, If you're going to drop $650 ( a fairly high price for a "labor only" compressor replacement IMO) into a compressor swap on a unit over 4 years old, you might consider replacing the entire outdoor HP. New unit, new 5-year warranty, new everything. Get prices and consider the -next- compressor replacement will be on your dime; over $1K. Most importantly, have somebody qualified try and figure out what caused the failure in the first place. Dirty indoor coils, poor maintenance, low air volume are a heatpumps worst enemy. Good luck with it.
Randy