Yah, usasma, that seems to be problematic when you can't even access the 2nd half yet!
The reason 4th edition was so radically different to the 5th is because Vista internals are completely altered compared to XP. An entirely new OS structure is going to give good cause for a vastly changed book to reference it. Windows 7 does have its own set of changes, but they're more like improvements or fixes to structural blunders they made in Vista and is therefore not much different as a result.
I've contemplated on getting it, but I believe I can hold off til part 2 pops up, since part 2 covers very important items in OS structure (like the I/O system). However, I don't believe any of it has anything that hasn't changed from what's covered in 5th Edition. The changes Windows 7 made from Vista are primarily on thread scheduling and object handling, which are covered in Part 1 of 6th edition of the book.