DJI Zenmuse X9 Counterweight

Yes DJI Zenmuse X9 Counterweight will work with the DJI Zenmuse X9 air as well.

The DJI Zenmuse X9 has a maximum payload capacity of 2.29 pounds after the weight of the gimbal itself is considered, so with the DJI Zenmuse X9 Counterweight attached, the remaining payload capacity would be 1.64 pounds.

DJI does not list a specific lens weight that the DJI Zenmuse X9 Counterweight is necessary for, but recommends the counterweight when using larger lenses that may be more difficult to balance.

A smaller counterweight is included with the Ronin 4D that can be attached to the bottom of the camera unit, just under the lens, in order to balance some lenses. The problem with mounting the smaller counterweight is that you have to choose between mounting either the counterweight or the optional external focus motor because they both get mounted on the same spot. I have the Sony 50mm F1.2 GM lens and the small counterweight that comes with the 6k package is nowhere near heavy enough to balance that big of a lens so I had to order the bigger counterweight to compensate. I wish DJI had done a better job of explaining what accessories are needed for various compatible lenses. I would have never wasted money buying an external focus motor that I didn't need and bought the bigger counterweight instead. Don't look to DJI for answers, their customer service reps have no clue and said they needed two weeks to get back to me with answers.

Yes, you can easily source M3-6 Countersunk Screws online for use with the DJI Zenmuse X9 Counterweight. Four of them come with it, so this may not be necessary.