It is possible you lost the 4th gear input shaft. Get the car scanned to see if any codes come up. Some trans codes do not trip a light. After that it'* drop the bottom pan to see if there is any metal there.
Take a look at the build date on your car. Likely it was a late 2002 build. GM started hardening the 4th clutch hub shafts in 2003. However I have seen a few late 2002 builds for the 03 year that did not have hardened shafts.
Likely your 4th clutch hub shaft looks like this.
It is possible to lower the cradle and fix this issue. It'* just a bit of work.
The question I have concerning the change out of 4th gear shaft.
If you lower the driver side to get at the side cover and the other vaqlve components, what keeps the 4th clutch held into place when returning the valve housing. They would appear to be quite slippery and slide off the hub before one could get the valve assy back into place. any comments>??
What gaskets are necessary to do this manuever?
You may have internal damage but I would bet it isn't. Overdrive kick-out plagues these transmissions. I'm not really sure anyone besides me knows how to fix this problem. I dealt with it for two years before I figured it out.
Heat. Thats whats causing your problem. The transmission restricts fourth gear when a certain temp is reached in the tranny and its reaching that temperature before you reach 45-50 mph where fourth gear is reached. This is the fix; Change the transmission fluid and filter. Put half a bottle of Lucas trans. product in during the change. Check that your engines cooling system is in top notch, since it directly effects tran. temp. being that the transmission fluid is cooled in the radiator. This is the key though, installing an external transmission oil cooler. after market ones aren't that expensive and thats the true fix, pulling out that extra heat. This works, in fact my car has never run better.