Original: The contract stipulated that a four-cylinder Ford-based F2 engine would be developed as proof of concept (see the FVA above) and that a pure Cosworth V8 would be built based on this.
Simplified: The contract told Duckworth to build a four-cylinder Ford-based F2 engine to prove it would work (see the FVA above). After that, a pure Cosworth V8 would be built.