Approximate cost to repipe 4 bdrm 3 bath with PEX in Los Angeles

No one can possibly guess based on your one line question.
The ONLY WAY is to ask 2 or 3 plumbers to give a quote, based on LOOKING at your house in detail.
PEX isn't very expensive. Fittings are a bit expensive. TIME is very expensive, and time is the variable that no one can give you without seeing your house.

Joy's correct! We can't give you a estimate without being on location to make a material list and lay it out. Also to determine the hourly rate of pay and the number of plumbers on the job. Get 3 estimates from local contractors and go from there. Good luck, Tom