I finally had the time, money, and ambition all at the same time, so while I installed a new piston in my '06 YZ450, I split the cases and swapped out 4th and 5th gears for the 4 and 5 from an '08 ('07-'11) WR450F. It was a straight, simple bolt-in operation in every respect, and except for the 3 snap rings that one should replace, the 4 gears themselves are the only required parts outside of gaskets and the like. The original shift forks get used if in good condition. Nothing to add.
It works out extremely well in the open desert, too. The ratio spacing through the first four gears with this setup is the same as the '03-'05 four speed exactly. Then, there's a nice step up to a real 5th gear that you can run with. I was breaking in the new top end so I didn't want to run it too hard in the first place until after an oil change, and the lake bed was cobbled pretty badly by all the recent rain, but I hit 76 mph with not much effort at all.
Shifting through the gears with a YZ450 5 speed, the gaps get progressively tighter as you shift; 1-2 is the widest, 2-3 is closer, and so on. This works good on the track because the top 3 are close enough that if one's not quite right, the other will be perfect. But out in the more open environs of the desert, it's like there isn't enough difference between 3rd, 4th, and 5th to bother with. With the WR top two gears, the gap between 3rd and 4th is about the same as between 2nd and 3rd, with the 4-5 shift being a little wider again. If anything negative can be said about it, the biggest mental adjustment to be made is to remember to go fast enough in 4th to make 5th usable.
But it pays off big when things flatten out. The bike will now cruise fire roads at 55-60 easily without revving the engine beyond its comfort zone, and it's no longer necessary to twist the thing to its limit on long runs over lake beds and such. With the Rekluse clutch to make up for the tall first gear, it's a near perfect ratio set for the kind of riding I do.
The part numbers for the WR 4th and 5th are:
4th pinion: 5TJ-17141-20-00
4th wheel: 5TJ-17241-20-00
5th pinion: 5TJ-17151-20-00
5th wheel: 5TJ-17251-20-0
The same swap can be done to the '10-'13 model YZ450 as well, but that requires a new output shaft from the earlier '06-'09 model along with the gears.