Ya, that don't sound like it'* in 4th. I would expect about 2500 or so. All other gears work? I'm not as familiar with the 4t80, but if it'* anything like the 4t65e, the 4th gear input shaft can strip out and that will loose 4th. Dropping the trans pan to change fluid and filter should show if that happened because there will be metal in the pan.