The FX9 can shoot 4K 120fps only when using an external recorder like the Atomos Shogun 7 paired with the optional XDCA-FX9 extension unit. Internally, it caps at 4K 60p.
Using a standard BP-U35 battery, the FX9 provides approximately 54 minutes of continuous recording. Using a larger BP-U70 battery extends this to roughly 120 minutes.
Yes, the FX9 features Dual Base ISO (800/4000). The 4000 ISO base allows for exceptionally clean footage in low-light environments without significant noise.
It uses a locking Sony E-mount. This provides the versatility of E-mount lenses with the added security of a breech-lock system, preventing lens play when using heavy cinema zooms.