I have read some mixed messages about 4M Express, why I just want to add my experience:
In order to save a day - and an over night stay in Arequipa - we wanted to go directly to Chivay for a Colca canyon trek. As 4M Express is the only bus company offering this route, we decided to pay the usd 50/pax and bought the tickets at the bus terminal in Puno. We asked about the size of the bus and was promised it was a large, modern turist bus. We also asked about any additional cost and was promised that there would be none. The seller promised that we would be picked up at our hotel in Puno the next morning at 5:50.
We were picked up (as promised), it turned out that we were the only 2 pax on the bus. The bus was a nice, modern bus (as promised) and there was an english/spanish speaking guide on board.
We had three stops on the way (baño + photos), a cup of coffee/tea/mate and a snack on the 2. stop (all good quality). The guide knew a lot of stuff about Peru/the region/the nature, so she was just fine as well. We arrived at Chivay at 12:00 and was dropped of at our hostal, which was very close to the bus terminal. We did have to pay 70 soles/pax for entry into the Colga Canyon park. This is inevitable, but I wish we were told up front.
All in all a very good experience with a good driver, a good bus and a good guide. Whether it is the usd 50 worth is probably an individual decision, but for us it was OK to avoid Arequipa.