The ND4 filter allows the passage of 1/4 of the incoming light, equal to a filter of 2 stops.
The ND8 filter allows 1/8th of the incoming light to pass, which is equivalent to a 3-stop filter.
The ND16 filter allows the passage of 1/16 of the incoming light, equal to a 4-stop filter.
ND4 × 1 filter
ND8 × 1 filter
ND16 × 1 filter
Mavic Air