My fourth gear failed not directly from power.
What happend was my drive chain (100,000 on the clock) became very streached from going to the track, and just general useage. At some point durring my trip I got on the throttle a bit before I exited the freeway.. the chain proceeded to flap the slack enough to smash the 4th clutch piston seal and blow it... causing the transmission to not be able to build proper pressure to shift into 4th.. the fluid just goes right past the piston.
The car is completely driveable.. works fine in 1-2-3.. put it in D and it won't do anything, just stays in third.
Don't worry about leaving your car in D for the track or street.. that isn't the issue in my case and many cases. HOWEVER, there is a significantly LESS amount of clutch for 4th gear, what I do recommend is if you are going to do a power pull on the freeway or just mash the throttle for giggles, manually drop it into 3 before you do. As when you first floor it, your power goes right through the 4th clutches to start, and can burn them out easily. I have done that a lot, we will see what they look like.. but when I last checked the fluid, I don't think it could be considered 'red' anymore LOL
-= 2006 - Panasonic 8401U HU - 2 12"MTX Subs - MB QUART dB6500 6.5" components - MTX 2150x AMP - Kenwood 7202 AMP =- AMAZING 1.5L 109HP Manual 5.. :P