the 1980's Finland developed their own class of racing based on Super
Vee, it allowed for a lighter chassis powered by 1600cc engine and was
called Formula VW 1600. When VW stopped supporting the category it was
renamed Formula 4 and the 1800cc engine from the VW Golf was introduced.
As the class became popular many second-hand F3 cars, especially Ralts
and Reynards, were converted to F4 specification as the two classes were
very similar in performance.
Thanks to Antti Rinne for additional information.