I've been shooting the sx3 for 8 years. Solid and reliable gun. Appears the sx4 is a cheapened up version although they try to market the few changes they made. Heck, even retails for less than it's predecessor. I've always loved the sx's, having several 1's, the 2 and 3. Nothing I've read about them wants to make me run out and get one.
Brother got one a couple weeks ago. It is black 3" model. You can tell where they saved money and called it weight savings or more ergonomic. Has same proven gas system.
That said it shoulders well, is light for gas gun, and the thinner grip in forearm and pistol grip fells good. It is a heck of a gun for $650.