Why does this show a 4 cylinder engine firing as two pairs? My ignorance is on display, surely it's more subtle and there are 4 distinct phased firings not two?
There are four distinct firings. Look for the little yellow poofs. The pistons move in pairs for balance (vibrations). Each piston is doing one of the four strokes (suck, squeeze, bang, blow is the crude way to remember) at a given time. None of them are doing the same thing at a given time.