4th gear isn't always best for gas mileage. The best thing for gas mileage is to keep the engine in its "powerband" (peak torque for the 4.0 OHV is at about 2400 RPM). If you shift into 4th gear too soon, then the engine is running significantly below it's powerband, which actually hurts gas mileage. It also creates a strain on the transmission, and the A4LD is already considered a weak transmission. Don't really want to stress it too hard. That said, 45-50 might be a little late for the shift up to 4th gear. See what your engine RPM's are running at before making that determination.
Also, make sure when you say OD, you aren't really referring to torque converter lockup clutch operation. I see a lot of people who confuse the two. OD=4th gear, not TCC lockup. For that matter, overdrive refers to any gear ratio less than 1 -- where the output shaft of the transmission is spinning faster than the input shaft. TCC lockup "replaces" the normal fluidic coupling between the engine and the transmission with a more efficient mechanical coupling. This is another part of getting better gas mileage from an automatic, because a significant amount of gas mileage (a couple MPG) is lost in that fluidic coupling. In my opinion, you might be better off in 3rd gear - TCC locked than in 4th gear - TCC unlocked.
After all this, if you still feel that the 3-4 shift is late (or the TCC lockup is late), it's pretty easy to modify by installing a "manual" switch. The 3-4 shift and TCC lockup are electronically controlled by the PCM. The PCM acts as a ground side switch in the control circuit. Simply install your own switch to ground, and you can then decide for yourself when to shift into 4th gear and/or when to engage the TCC lockup clutch. I remember a detailed discussion here about making this modification several months to a year ago.
