On MB transmissions 722.4 a faulty teflon seal at the K2 clutch will cause the car to lose 4th gear. I've seen this many times. I'm not sure about the hissing noise in reverse. This requires removal of the transmission and nearly complete dis-assembly. The longer you drive it this way, the more damage you'll do to the K2 clutch discs and steel plates. I did this job as a favor for a friend a few years ago, and the parts cost me about $300, but I don't know how much a re-build goes for these days. Although I work at a dealer, I don't pay much attention to the cost of parts and labor. I'm told that the price of re-built units has come down considerably in the last few years. Perhaps some other members can help?