Not sure if air con was an option on 4/4 Barry. The 4/4 wiring schematic shows no wiring for such in chassis loom unlike +4 and Roadster. Having said that the fourth switch hole is probably fitted with a redundant switch just to fill hole as you suggest. In this likely case it could be used to control something else - front fog or spots for example.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone