After the WoTC trauma suffered in my circle of friends with 3.5 (come on, what is it? a computer program? At least with those you get a discount to upgrade) .. I'd be a little unhappy with a 5thRevised/Updated/Whatever book.
You know what'd I'd like to see? An Adder book (heh, even fits the game mechanic). Not yet another full rule book.
Just take all the Errata, the FAQ, the Genre by Genre section, a few sample characters and put it in a book, call it Fifth Edition Adder and sell it moderately priced. Or even exorbatantly priced. The last thing I want is one more outdated complete/full rulebook sitting on my shelf catching dust.
And so you can satisfy those who don't want to buy two rule books just to start playing put out a full printing of the revised if need be .. but please, just don't make me pay for a complete set of rules again so you can fix some typos and add a bit of errata. Really.
Make the Adder Book a soft cover, or better yet offer up a PDF version I can buy online .. anything but a complete edition of the rules again.
that's my opinion at least.