Tought a4 was 316, just another name, if you want higher quality go for a5, but a prop shaft should be ok at a4, a really though stainless is a feric one (f1) but machining it then becomes a problem, these figures relate to the tensile stress/elongation of fasteners , not a property of a prop shaft where the torque value is more relevant,the only reason to make a shaft out of stainless is to stop it rusting,a steel shaft is more than strong enough for the job...but rusts.
Some stainless steel ,is not suitable for salt water use because of it s chemical make up...hence brown staining on cheap rails and screws, 316,a4 is recommended for marine use(salt water aplications)