The firing order is 1-4-3-2. Looking at the engine from the rear (through the engine compartment).

3 1
4 2

These numbers are written on the tinware by the sparkplug holes of the Type 4 engine. The firing order is also written on the tinware above the ignition coil, and on the Type I engine it's stamped on the alternator stand. You will hopfully see (if your tinware isnt to rusty), the word Zundfolge which is German for "firing order".