That was good advice about dealing with the emergency of being locked out of 4th (O.D.). But I can't recommend permanently bypassing the relay, or modifying the solenoid, to lose the ability to manually select between 4th and 3rd gears.

Please (for the sake of 240's, and 240-lovers, everywhere), for preventing their premature demise, which makes them scarcer and harder for us to find good ones) consider:

Unless you live in really flat territory, never pass other cars, and never have to merge onto highways, it's really handy (and better to continue to allow one) to use that button to shift down (as you give a little more gas pedal, to better synchronize), and the main advantage is to allow a somewhat more gentle downshift (as when you're getting ready to pass on a 2-lane road, or you're climbing a steep hill (or descending one).

The alternative would be to floor the gas pedal, which would also downshift from 4th but at such high torque that you're really abusing the transmission -- ironic that folks are concerned about somewhat higher revs (if the relay fails and you're stuck in 3rd), but doing only full-throttle downshifts seem not to be another matter of concern.
Also, in ascending hills, you might be in that in-between zone (maybe the car ahead is too slow?) where your transmission is constantly shifting up and down, again and again -- it's far better to hold it in 3rd.
And certainly the ability to hold it in 3rd when descending long grades is obvious.

I think that the advantages of maintaining the manual control of 4th-to-3rd downshifts the arguably rare case of electrical glitches (relay, wiring insulation, etc.).

Certainly, the emergency repair of jumping the relay's wires is a valuable knowledge, and I will make note of it for the next time I have to use this helpful piece of advice -- I'm grateful for being informed -- but I suggest that one carefully consider the impact of permanently defeating the feature before doing so, especially such an extensive modification as the one involving the solenoid.

Best regards to all.