B&H has a 77mm 4-stop ND filter as it's Deal of the Day today for $44.95. It is a slim profile which is better for wide angle shots (less vignetting). Looks like 50% are already claimed and most likely will sell out early.
Hoya 77mm Pro 1D 16x (4-stop) Neutral Density Filter XD77ND16
Hoya 77mm Pro 1D 16x (4-stop) Neutral Density Filter XD77ND16