The 4-prong headlamp switch is for later 240's which used a relay to handle the headlight power demands. It's a good thing it won't fit the earlier harness connector as it wouldn't last long.

Our 1983 melted its plastic harness connector, too. I found a junkyard unit which had an only-moderately-overheated harness connector. I have yet to see one in perfect shape. I installed a 30-amp relay near the headlamp beam switching relay, which gets power from a 12-ga wire straight from the battery's positive post. This means the headlamp switch only handles the current required to run the parking/tail/licence/instrument lights, much like the newer 4-pin switch setup in 240's (and 700's)