For those obsessing about my comment about terra firma, it was a simple wisecrack in response to the post of another. Sheesh.
As I described earlier, measure the pulse width of each of the four signals and record those if they change beyond a predetermined deadband. Do that round-robin, saving each change to SD, along with a millis() timestamp, if it's warranted. You must determine what is a significant change, and how frequently to measure and save.
Not optimal with an Uno, as your pulse width measurement will be 4uS increments, iirc if you use micros(), not sure what can be done if you use a hardware timer, but it's a start.
Good luck, @rcmodeller.