The lower turret slot is listed as a S4 turret. Does that mean it has a S6 slot fixed / S5 gimbal? Or is it hard mounted as a turret and the max size is S4?

The underbody hardpoint is a S4 hardpoint. This would allow for a single, fixed S4 weapon, a single gimbaled S3, fixed twin-link S3, or gimbal twin-link S2 weapon configurations.