Maximum CPU posted a review on the Microsoft SideWinder X4 Keyboard

Modern keyboards (like the one on my Macbook Pro, that I am using to type this) have a moderate degree of redundant circuitry to allow for keyboard commands, but still fall short of some selective usage scenarios. I can easily press a few different keys at once (and elicit the appropriate response), but pressing four keys arranged next to each other in a square will only report the first three. Video games have emerged as the group of applications that have the fastest combinations involving the most unique keys. For these programs, users are not aware that their key-presses are going unreported, and will often assume the program or their skill is at fault. To address this problem, Microsoft has developed more advanced circuitry in their SideWinder X4 to allow for more simultaneous keys to be reported. Additionally, this technology allows for any of the QWERTY keys to be used simultaneously, and is cheaper than addressing each key with unique circuitry.

Microsoft SideWinder X4 Keyboard Review