It seems that the next generation GT4 will continue with 6 cylinder engines according to this Autocar article which also reports that the GT3 will have a manual option but that GT3RS will continue with PDK.
"Contrary to those who maintain the new 718 Cayman will only accept four-cylinder engines, Preuninger said there will be a new Cayman GT4 and it will have six cylinders."
http://www.autocar.co.uk/...-manual-gearbox-option
"Contrary to those who maintain the new 718 Cayman will only accept four-cylinder engines, Preuninger said there will be a new Cayman GT4 and it will have six cylinders."
http://www.autocar.co.uk/...-manual-gearbox-option