Those are VERY expensive, and I doubt he cares about 4k, sounds like he is trying to do this on a budget (Though a 400" screen on a budget is tough, it is possible). We don't have nearly enough information to even be answering this post realistically, as it depends what the expectations are on quality and the budget. That said, here is the most complete answer I can give...
None of the super high lumens projectors are really home theater projectors anyways. Double stacking is the cheapie way,Double Stacking on the Super CheapSub $5500
The Epson PowerLite Pro G5450WUNL is about $2500 and rated at 4000 lumens, or 8000 double stacked lumens (7000 real world), and combined would produce 6000 lumens in a decent looking mode when double stacking them. The total cost would be $5000 after buying 2 units for stacking. Since this is an LCD, it has no RBE. For a few more thousand you could double stack the 5000 lumens version instead.
The cheapest possible OK option I can think of is (2) Optoma eh501's give a rating of 10,000 combined lumens for about $3,500 total, and I think they have enough have lens shift for double stacking. The major issue with this will be RBE. Also you will not get anywhere near 10,000 lumens calibrated, but I assume a combined about 7000+ lumens or so in a very watchable close enough (though not perfect) color mode.Sub $2500
Instead, you could also get (2) Benq sh910's for about $2500 total which would provide 8000 rated lumens double stacked (7600 measured), it has an ok mode to give you around 6500-7000 lumens with presumably good enough color. This would probably need to be a bit of a keystone hack job for the the double stacking though. There is a review on Projector Central for this PJ. The sh910 might (and I stress might) have less RBE than the Optoma eh501, but it doesn't have lens shift like the two above options, which is why you'd need keystone to double stack.Sub $1500
Even cheaper would be (2) Viewsonic PJD7820HD, they only cost $700 each and they put out 3500+ measured lumens, giving you 7000 total stacked lumens @ 6527K gray-scale, though the sat and gain of Red will be a bit under-saturated looking in this mode. This would require keystone, but it's possible. The total cost would be $1,400 (7000 real world lumens for $1,400 haha). Again too much RBE MIGHT be the main issue for a large audience. Even though the Viewsonic is only rated at 3000 lumens, Art @ PR
measured it at 3700 lumens in brightest mode at closest throw (3518 mid zoom) and Evan measured it over 3300 lumens, so I think it is safe to say this PJ can pump out 3500 lumens on a new lamp with no problem. As much money as you save on this option, you could buy 5-10 spare lamps so that this projector is probably the brightest cheap option economically.Single Projector Solutions
For a single projector 1080p solution, the EIKI LC-HDT700 (LCD) is rated at 7000 lumens and costs $11,000 street.
The Epson PowerLite Pro Z8455WUNL (1920x1200) is also rated at 7000 lumens for about $10,000 street.
The Epson PowerLite Pro G6750WUNL (also 1920x1200) is rated at 6000 lumens for about $5500 street,.
There are some other lower model Epsons in the same line that provide 5000-5200 rated lumens for under $5000 street, but you are getting too borderline on the lumens at this point, unless you double stacked.
There are several Barcos, Christis, and other EIKI's near the same price, but not sure if you can get that many lumens at 1080p+ for that same price range.
For a 720p projector (actually 1280x800), Panasonic makes some DLP's with 8500 lumens for around $7,500, though I doubt the contrast will be any good.