If you are determined to film in 4k I personally would film in 4k 30 over 24 as 24 frames per second usually yields sort of stuttering footage, a lot of people say they don't see it but I always see it, no matter how smooth you can fly, especially while panning. As far as using ND filters, if its really bright put on an ND 16 filter or even 32 if you have to squint because of brightness. Film in manual and keep the ISO at 100, if you are filming 30 fps then set your shutter speed to 60 (just double your frame rate.) Once you put the bird up check and see if your exposure is correct. If its too bright or too dark bring down the bird and change to the appropriate ND filter, ( you can adjust slightly with the right scroll wheel, however if you go below 60 on the shutter speed you will get to much motion blur) Eventually you will be able to get it right the first time. Unless you are planning on color correcting and grading I would shoot in Normal mode instead of D-log or any of the baked in presets such as film or beach. If you want to just do a small amount of color grading you can get really nice results filming in d-cinelike. Use +1-0-0 ( theres plenty of threads on DJI's noise compression below +1 sharpening) if you are new to premiere pro check out youtube tutorials from a guys called chin fat. very helpful and free.
oh also leave the gimbal clamp on while putting on or taking off ND filters but don't forget to remove it before the flying! I am the one who sold OP this Mavic so I know it like the back of my hand, Have a blast on vacation man she is a real joy to fly!