The car will start off in 2nd gear, so when you count from start, there appear to be only three gears. And the final gear will appear to be way too low, because the RPM's will appear to be way too high. That's how the car was designed. When I first test drove a W113 I thought the 4th gear was broken also. At 65 MPH, I guess the RPM's would be around 4,000?
First gear can be engaged by putting the selector in "2", or by (almost) flooring the car from standstill (that is, if the automatic transmission is in good shape and all of the linkages, 3-position solenoid, microswitch etc are all adjusted properly). But you will notice the car will shift into second gear real quick anyway.
Cees Klumper in Amsterdam
'69 white 280 SL automatic