Day-one essentials: (1) microSD card — the Mini 4 Pro has no internal storage; without a card you cannot record. Get 64GB Samsung Pro Plus minimum, 128GB is better. (2) FAA registration at faadronezone.faa.gov ($5, lasts 3 years) if you fly a drone over 0.55 lbs. The Mini 4 Pro is sub-249g so recreational registration is not legally required, but verifying Remote ID is active in DJI Fly app settings is required.
Buy-soon (within first week): Extra batteries via Fly More Combo (one battery = 30 minutes then a long wait). A 65cm landing pad ($15-20, protects the camera from debris and moisture on every landing).
Buy when ready to improve video: ND filters — learn to fly first, then add filters when shooting intentional video. A quality carrying bag — the stock pouch is adequate while learning.
Skip as a beginner: Propeller guards for outdoor flying; range-extending accessories; ND filters before you understand camera settings. Buy DJI Care Refresh ($79-99) — the first year is your highest-crash-risk period.
Check Drone Starter Accessories on Amazon