There is a lot of misinformation about engine efficiency and BSFC. Where is the engine supposed to be "more" efficient? Overall? At cruise speed? During stop n go city traffic?
The A, Number 1 factor affecting a conventional engine's BSFC is piston ring swept area. The lower the piston ring swept area, the more efficient the engine.
A 4 cyl eng with 350 cubic inches has far less piston ring swept area than a similarly sized V8, due to simple geometry. In addition, that 4 cylinder has far less combustion chamber surface area to transfer combustion heat away from doing useful work. The 4 cyl will achieve a peak BSFC of 0.42-0.45 Lb/HP/Hr, compared to the V8, which will achieve a peak BSFC of 0.52-0.55 Lb/HP/Hr.
Today, the Prius achieves a thermal efficiency of 40% (the most efficient production engine) , or a BSFC of about 0.36 Lb/HP/Hr due to various non standard operating modes and high rates of EGR flow.
Of course, a typical V6 has fewer main bearings than an inline 6. And a flat 4 has only 3 main bearings. Light aircraft often chose a flat, air-cooled, 4 cylinder of large displacement. The combo, designed in the 1930's is rather efficient, nearly matching the prius engine, with a peak BSFC of 0.38.