Greg @gkittinger - here we go;
Having checked a few of my files, first of all, I did remember incorrectly; the Friedrich did not have pressurization. That was from early G-models. You are of course correct in removing the air intakes for cooling the sparkplugs.
As goes for the F-3 some claim 15, others 20 planes were made, and they have been difficult to trace. The F-3 and F-4 did use 87-octane B-4 fuel (the F-4Z with GM-1 nitrous oxide addition), but the main difference is in the Motorkanone! Externally the F-3 and F-4 are very similar, but the F-3 used the MG-FF/M as the engine-gun, and the difference is thus mostly to be seen in the cockpit with the large cover for the bulky drum-fed MG, the F-4 using the MG-151.
As goes for the strengtheners on the aft fuselage, they were often field-mounted on those that did not have them from the factory. The F-4 mostly had them from the factory.
Many F-4s were completed with the round wheel wells as seen on earlier F’s, and the major difference is the air intake for the supercharger with all F-4 being the “fatter” type, earlier planes usually had the smaller round ones, the square type being typical on WNF built F-1’s (these also had a steel-plate wheel in many cases). Those F-4's with round wheel wells were re-manufactured from repaired early F-2's and F-1's, and no matter how you define them, all surviving early F’s were rebuild to F-4 standard as they returned for major service. So in conclusion, if you choose round ones for a F-4 it is no less correct than squared off ones. The real clear identifier is the fuel-triangle; if it is labeled for 100-octane C3 it is most likely an F-2. (contrary to some claims that high-performance engines need 100octane the DB's were using 87 octane in the DB60E, the DB601N needing 100octane)
