John's number of 17 is applies to an operating, correctly charged system. This number is the high side pressure of the system, expressed in atmospheres (bars) if memory serves. The value 17 equals about 255PSI, which is a pretty normal high side.

Your value of 4.0 is about 60PSI. Whether that's a problem or not depends upon the temperature under the hood at the time you took the reading. What was the ambient temp? Had the car been driven before you took the reading? Sixty PSI is low if the car was cool and the outdoor temperature in the 80s or 90s, as I expect it is in Fl. If the car had been driven, making everything under the hood nice and toasty warm, then it's really low.

I kinda doubt a 4.0 reading is low enough to cut off the compressor. Typically it would need to be something in the range of 2.0-2.5 to shut down the compressor. Not sure if low refrigerant is your main problem or not.

- JimY