Javascript

As a novice I'm trying to create Javascript for a PDF whereby ticking boxes 1, 2 or 3 automatically ticks box 4 but I want the tick in box 4 to be unticked if boxes 1, 2 or 3 are unticked/deselected.

At the moment box 4 remains ticked if 1, 2 or 3 are deselected.I've tried options on the net and the code at the moment is if (event.target.value!="Off") this.getField("TICKBOX4").checkThisBox(0, true); but I can't work how to automatically untick box 4 if any of the other 3 are unticked. Any help would be appreciated and I suspect this is quite straightforward if you know what you're doing!!!