Four hours sounds a bit steep. I can have the expansion valve in & out of my 124 wagon in about 30 minutes - it just ain't that difficult a job. Of course, there's the matter of recovering the refrigerant first, vacuuming the system, and recharging - all will add time. But four hours sounds high to me.
How does the sight glass on the dryer look? If it has become opaque rather than clear, you may have a clue to the problem.
The most likely cause of the low side showing high pressure is a dying compressor. Is the high side pressure too low?