I have a 4 ton Rheem unit and the condenser coil in the attic needs to be replaced the expansion valve is sticking and the coil is leaking freon. I wanted to know approximately what this is going to cost.

Depends upon the labor rate wgere you live. If they can get a coil for the old unit that will fit probably 1200.00 to 1500.00 with new expansion valva and a liquid dryer.

I would get a second opinion. That does not sound right. I have been doing AC work for 35 years and have only seen one evaporator coil (not condenser coil in attic) that could not be repaired easily. They usually leak from a capillary tube allowed to vibrate or two pieces of tubing rubbing together. An expansion valve can be replaced for about $150 plus labor and freon which should be about $500. You can get a whole new air handler for about $1000 for that model. DrTom4444