The exact same thing happened to me, and It was the clutch pack on 4th gear. 3rd was on its way out. The car would shift into Reverse, drive, and 1/2/3 fine, but 4th would just not engage. 3rd would also occasionally not engage while this was happening.
My mechanic dropped the pan, and found the clutch packs were gone for fourth gear and third was on its way out.
Unfortunately My mechanic had to rebuild the tranny.
IT was not too expensive compared to the MB Dealer, and I had other stuff done. I figured the tranny apart from the other stuff was around $1500 to get done.
I hate to be the bearer of bad news, but you just might be in need of a tranny rebuild.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.