You'll rarely need an ND 4, and often will need more than ND 16. Freewell is coming out with an ND/CPL combo filter set of 8,16,32,64 which are priced right and are multi-coated. Ordered mine today.
https://www.amazon.com/Freewell-DJI-Mavic-Air-Filters/dp/B079BS17X2