Ducati ST4S Owner's Manual page 194

Number plate light
To expose the number plate bulb, withdraw the bulb
holder from the number plate holder,
then extract the bulb and replace it.
E
Stop light
To replace the stop and parking light bulb, unscrew the
two screws (1, fig. 88) that secure the glass and remove
glass. The bulb is of the banjo-type: press and rotate
anticlockwise to remove; fit the spare bulb by pressing and
turning clockwise until it clicks (fig. 89). Refit the glass and
tighten the screws (1).
84
1
fig. 88
fig. 89