The Leatherman Juice and Squirt are good keychain-sized multitools. If you want something more medium-sized, the Skeletool and Style are solid as well. When I carry a multitool, it's usually the Skeletool CX. Has all the tools I need and nothing I don't, and it's small enough that it rides comfortably in the watch pocket of my jeans.
The size you described (4" closed) is a full-sized multitool though, so you could even go with something like a Wave or Charge.