|I think an ND4 filter is actually a 2 stop (maybe wrong on this one), but to be honest I found that I was still having to underexpose by quite a lot and I think I woudl have been better off buying an ND8 filter.
Found this article
"For example, Tiffen and B+W have 0.3, 0.6 and 0.9 ND filters for reducing one, two and three stops of light. Hoya, on the other hand, uses 2×, 4× and 8× to indicate reducing 1 (i.e. 2=21), 2 (i.e., 4=22), and 3 (i.e., 8=23) stops. The two ND filters come with Nikon's Coolpack are of ND4 and ND8 type, which means they reduce two and three stops, respectively. "
Hope that helps. I'm only just starting to use the ND filters, though have had a grey grad for longer, I have found that the grey grad was also not strong enough - so either I don't know when to use them, or don't use them often enough...
The Rock (50) *