Nothing wrong with your English! I think these discussions are always made more difficult in the need to type an answer, and the time lag required to answer simple questions. Much easier in the pub, over a drink, and a lot of finger tapping on photos.
I think your proportion method is a good start, though I'd be careful using it alone (without other cross referencing) on crashed aircraft in general...with possible crushing of structure. Your explanation of a Dornier 17 (perhaps one of the early E or F 'flying pencils) seems quite reasonable.
I have seen German '4' as in the closed 4 but with a small line/space/demarkation between the < and the top of the |, perhaps a stencil connection, (though I admit to being a novice with most things Luftwaffe). I can't quite make out the finer detail of the 4 in the photo.
I'm happy to leave it to the 'experten' to decide.
Regards,
...geoff