Where do you live? Is it really cold there right now? Your transmission isn't supposed to shift into overdrive until the engine is warmed up. There's a special sensor on the front of your thermostat block especially for that purpose. I think it's the one on the front of the block furthest to the right if you're standing in front of the engine; it has a single wire attached to it. Anyways, that's the sensor that, once warmed up, allows the automatic transmission to hop into 4th. If it seems irregular to you, clean off the lead with a scratch pad and reattach the wire. If that fails to please you, buy a new sensor and screw it in.

I think this is it:
P/N MD102329 - Switch unit, water temperature
and retails for about $50. Kinda pricy, but far less than rebuilding your transmission.

You can also remove the switch and test the resistance at cold and and operating temp to see if it has gone bad. I'm sure Frank will follow up with the correct proceedure and numbers. <img src="/forums/images/graemlins/wink.gif" alt="" />

Oh, and btw - for $3000 you can buy a new transmission from Mitsubishi. You can get a rebuilt unit at Autozone for just over $1100 with a $450 return core charge... if I understand correctly, that's just over $650 for an essentially new transmission with a one-year warranty if you can do the installation yourself!