I have no idea what the "book" is on this job, but I can see four hours easy.
You have to locate the problem, then evacuate the system, repair the problem and possibly replace items like the drier (which should be done whenever the system has been open to atmosphere, such as with a large enough leak). Then once all that has been done you have to evactuate the system again and draw a deep vacuum, like 29 inches HG and hold it for a time, then recharge and oil the system. Plus test the system afterwards. It all takes time. Sticking another tech on the job likely saved an hour or two labor charge.
Just this DIYer's opinion.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.