There are a couple of IR-based triggering devices available commercially, and I've also seen circuit diagrams for building one yourself. Sorry I don't have a link, but you should be able to find one with a Google on "circuit diagrams" and/or "IR triggers". Essentially, they amount to an IR beam the animal triggers by breaking the beam, and an associated solenoid that activates a cable release. The commercial unit included a housing into which the camera was placed. As the F4s has a manual cable release socket, rather than electrical, it should be relatively simple to construct.