This update prepares your PS3 to interface with the PS Vita, which would be really awesome if anyone had a PS Vita. 4.00 sets up the PS3 to copy games, save data and other media back and forth between the Vita and PS3. Firmware 4.00 also allows Plus members to opt in and out of several automatic update procedures, including patches, trophy sync, and recommendations.
It's not the most exciting update, since we don't have Vitas yet, but it's also not the least exciting -- that honor is reserved for updates that do nothing but address security.
Sony PlayStation 3 (late 2012)