In September 2005, Microsoft introduced the Natural Ergonomic Keyboard 4000 at a retail price of . This keyboard provides a significantly changed ergonomic design, including an integrated leatherette wrist rest, noticeably concave key banks, and a removable front stand to generate negative slope, which helps to straighten the wrist and allows the fingers to drop naturally onto the keys. The multimedia keys have again been redesigned, and the six-key nav/edit cluster has been fixed by returning to the standard 3×2 horizontal rectangular layout. The F Lock key now defaults to "on", providing the original function key features rather than the new "enhanced" functions, and retains its setting across reboots. The 4000 has the indicator lights for Caps lock, etc. moved back to between the banks of keys, although they are now below the space bar, rather than above. The 4000 is also significantly quieter to type on, with less of the distinctive "click clack" noise that is common with older keyboards; as it is likely uses cheaper membrane key switches (as opposed to mechanical), which tend to be quieter but have twice as much travel before depression. The space bar, however, has been reported by several reviewers to be unusually noisy and difficult to depress. The 4000 is available in two variants, Business and Retail. The exact differences are not known, although product descriptions imply that the packaging is different, and prices are often slightly higher for the Business edition.
