[Christ]

In orange4boy's case, there are only two solenoids, and a series of on or off for each one selects the gear that he's in.

Since there are 4 gear possibilities, and 2 solenoids, the simple math would say 8 microswitches, activated 2 at a time, in each gate of the "shifter".

Mill yourself a piece of metal, weld small tabs at each gate opening. attach two micro switches to each tab, in such a way that the "stick" would actuate them.

If first gear is OFF OFF, You don't need any switches in the first gear gate. 2nd is ON OFF, so you only need one switch there to turn the first solenoid on. This can be directly wired with a wire junction for more than one switch on each solenoid. Gear 3 is OFF ON, so you only need one switch in that gate, as well. Gear 4 is ON ON, so you need two switches in that gate, which, when activated by the selector stick, turn both solenoids on.

This would still require use of the original selector to put it in D, but no controller, other than making sure your TCU doesn't get pissed at you.