Hallo/Hej!

The transmission seems to be in fault mode, disconnect the battery to reset transmission ECU (minimum 5min). If it not works drive to Audi workshop for reading out the faultcodes for transmission. Check transmission oil level, special oildipstick required.
(Audi V8 automatic transmission stays in 4 gear if a fault is detected, after ignition is turned off and on the transmission stays in 3 gear).

Audi V8 4.2 (Panther Schwarz 92)