LLLL.com sold out, and there were 456,976 of those.
There are only 100,000 NNNNN.com, they cross language barriers so no concerns about different languages, IDN, etc.
Someone pointed out that one company owns about half of them all for use as US zip codes, so that further limits the availability.
They will be sold out within days and will automatically be worth double reg fee at least IMO.
They are good for use as a short, numeric domain, they are .com, and with 6 billion people on the planet, 100,000 names isn't enough supply (if you want the .com version). These could also be good for a mobile website address because the numbers are featured on a cell phone pad.
NNN.com and NNNN.com are appreciating, so NNNNN.com is the next best bet for a numeric domain.
Sure, it's all speculation, but hand regging any domain these days is speculation, even two and three word combinations.