This isn't fact, this is speculation:
The first rollercoaster that will break the 400' mark will probably be a Thrust Air 2000 coaster probably in five years. They have bragged (S&S) that their coasters will blast off faster, with more force, and at a much greater height than their prototype & with Hypersonic XLC.
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If that doesn't take place, then I would have to seriously say that if there will be a FULL CIRCUIT 400' coaster that will be built, it will most likely take place in around TEN years. I don't say this because of the time span between Millennium Force and Magnum XL-200.
I say this because of funds. Your funds for a coaster greater than 300' will start increasing exponentially. If you grow to 325, 350, 375, and 400 feet... your price for those coasters will be exponentially greater since you need a greater pull out, much more track, and much more space to accomodiate the speed and intensity of that coaster.
I'd think that a Thrust Air would be more likely to break the 400' barrier faster because the whole basis of the coaster isn't just the immediate thrust of air, but the initial vertical hill and verticle drop, which saves on a LOT of steel unlike your typical hyper (ie: coaster with no loops depending on speed for its main attraction) coaster.
That's just my speculation (overall well thought out guess, not off the top of my head!!!)