Nikon F 4 Instruction Manual page 40

Continuous Servo Autofocus
Use AF-L (Autofocus Lock)
button.
2. Keeping shutter release button lightly pressed, confirm in-
focus indicator LED appears, then depress AF-L button and
hold in.
1. Position focus brackets on subject and lightly press the
shutter release button.
3. With AF-L button depressed, recompose as desired and
fully depress shutter release button.