According to the guys in blue shirts working out of the factory semi at Washougal national, the overall gearing reduction for 4th on the new 450 is nearly identical to 5th on the 426. They have merely widened the ratios due to the increased power, more gears are not necessary(less shifting=better). They stated that the desert guys will still be using YZ's with higher overall gearing(sprockets). If there is going to be an issue, it will be the lack of a low enough 1st gear for the woods and the inherent stalling, not a top speed issue.
The gear ratios and internal reductions are available for the yz on the yamaha
web site. If we do the math, we see that the overall reduction for the new 450 in 1st gear is 18.6397 compared to 19.072 for a yz426 (2.1%higher) and 24.962 for a WR400(36% higher). What we are losing is the WR granny gear for the really tight stuff.
When you look at top gear, the 450 overall reduction is 10.146, which is about the same as a 426 with an extra tooth on the rear sprocket. If we assume an 11,000 rpm and do the math, we come up with about 85mph stock for the new 450. We can assume that it will easily pull higher gearing, with the limitations being how high of a first you are comfortable with.
I can see it as very similar to what we have done for years with off roading motocross machinery. 15 tooth counter in the desert, 14 or 13 in the woods. My experience as an expert level rider in the desert on a KX500 that would actually go that fast, is that only a very small handful of riders will ever use 85 mph on anything short of a graded or paved road.
I don't believe for a minute that the 4 speed was to save the half pound gear. It is about not needing the extra granny first for the intended purpose(moto-x), so why not remove it for a more compact engine or allow for slightly stronger gears that are used? I think that the moto-x people will rave about it, and we off-roaders will adapt with sprocket changes or by a WR...