Cyril Malem, when he ran CTG Racing, had some four valve heads made. As one unnamed well known clever person said, " It probably produces enough extra power to propel the extra weight down the road " I also think that George Spence produced a drawing of such a device, but I may be wrong. Cheers, ET43
http://www.vincentownersclub.co.uk/photos/showgallery.php?cat=782
Check it out !