The S1 autopilot by Raymarine is the replacement for the ST4000 wheel and tiller pilots. It is a 'below decks' pilot as to the computer and flux-gate compass. The tiller pilot comes with a steering rate gyro integral to the computer. The wheel pilot has the rate gyro available as an add-on. Raymarine appears to be moving in a modular direction, with greater compatability of their components.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.