Mitsubishi Repair: 95 Galant Auto Tranny Problem, loud thud, gear ratios

Question
If the fourth gear is shot, then why can I drive above 40Mph with the overdrive off?  If I turn off the overdrive then I can do highway speed with no problems, except that the egine RPM is much higher because the overdrive is off.  
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Followup To
Question -
My Galant can shift in lower gears with no problems. However, when it goes pass 30-40 Mph the RPM revs high and the tranny will not catch. If I turn off the overdrive then the car will run fine but the RPM is higher. I brought the car into a tranmission shop and they told me my fourth gear is gone.  The trans sometimes make a loud thud when it's cold when driving.

Answer -
Do,
If your 4th gear is gone, your car will have problems getting above 40 MPH.  The mechanic you brought your car has most likely diagnosed your problem correctly.  The only way to know the extent of the damage will be to open the tranny and inspect it thoroughly.  Get an estimate on a repair and then ask if it would be less expensive buy a rebuilt transmission.  Sometimes you'll have money in labor costs.
Good luck!

Answer
Do,
Your 4th gear is your overdrive.  In your car (and most auto transmissions) the overdrive is accomplished when the first 2 gears move together; making a 3rd and 4th gear.  But in order for the other gears to engage with each other, a series of clutches and bands need to activiate.  If the clutches and/or bands are not engaging the gears as they should, you will have no overdrive (4th gear).

So, when you have your overdrive on, you may have a planetary carrier or sun gear that is simply spinning free when it should be engaging.  This will prevent going above 40 while in overdrive.  But with your overdrive turned off, you have different gears engaging, which is allowing you to travel above 40 mph, but your RPMs will be much higher because of the gear ratios being used by your transmission.

Auto trannies are pretty complicated machines.  If you don't have confidence in my answer or what your mechanic is telling you, you should take your car to other transmission shops and have them diagnose it.  The more estimates and diagnoses you can get, the better.  You'll probably get better pricing that way.
Good luck!