What's that, four USB ports on the new Model B+? That's right! The new Pi has even more USB ports. The original B had a 2-port hub + Ethernet controller, the LAN9512, which basically turned the 1 USB port on the processor into 2 ports + Ethernet.
The new B+ upgrades the LAN9512 to the LAN9514 - a 4-port USB Hub w/Ethernet. This replacement makes it possible to plug in mouse, keyboard, USB WiFi and maybe even something else all at once.