the positioning/numbering of the cylinders is purely dictated by where they are bolted to the crankshaft, nothing technical or confusing about it.
as the con rods are bolted individually to their own crankpins (unlike say a rover V8) then they are staggered slightly, not perfectly opposed cylinders.
so, sat from the drivers seat (lhd, or rhd. i don't mind, they are the same for this explanation) then cylinder number 1 is on the right nearest the cam-belt, or radiator if you like.
number 2 is the left hand side one, again nearest the rad/cam-belts.
number 3 right/rear.
number 4 left/rear.
now, as the forward 2 cylinders go up & down the bores at the same time as each other and the rear pair do the same as each other it is physically impossible for the firing order to be 1-3-4-2 as that would need pistons 1 & 4 to be at the top of their own strokes at the same time, which they are not.
1 & 2 are at the top together, 3 & 4 are at the top together.
so the ONLY
physically possible firing sequences are either 1-4-2-3 or 1-3-2-4
anything else just can't work as the piston would be at the wrong end of the bore.
the firing sequence is 1-3-2-4. that's a FACT