i recently bought an ND4 neutral density filter and was expecting to be able to take quite long exposures in the middle of the day but after just giving it a quick test run it dosnt seem to block the light anywhere near as much as i expected.
if i was at the beach and wanted that misty look to the sea what ND filter would i need?
i dont think this one would give me long enough exposures to get the effect i am looking for.
i have read that people use multiple nd filters so i guess thats my answer?

Check this out: http://www.ephotozine.com/article/Using-a-...-Density-filter

You probably need at least 10 stops to get what you're looking for. I use a 13 stop filter if I'm doing daytime long exposures and quite often it's too "dense" - but not by much - so I suspect 10 would be enough for most situations. The more filters you stack the worse the vignetting will be.

Another problem is that different manufacturers use different terminology for their ND filters. For some, "ND4" means 4 stops, but for many of then "ND4" means only 1.3 stops darkening. Check the manufacturer's info about your filter. Brian is right, that about ten stops sounds about right for what you want.

No filter manufacturer in this world would call a 1.3-stop or 4-stop ND filter "ND4".

There are two ways of indicating the density of an ND filter---either by specifying the exposure factor (e. g. "ND 4×" which means two f-stops) or by specifying the logarithm of the exposure factor (to the base of ten) which is particularly common with the denser ND filters ("ND 4.0" means an exposure factor of 10^4 = 10,000× = 13 f-stops). Each f-stop adds 0.3 to the logarithm because log 2 = 0.3---so ND 4× = ND 0.6 = two f-stops. ND 8× = ND 0.9 = three f-stops. ND 16× = ND 1.2 = four f-stops. And so on ...

An ND 4.0 filter (i. e. 13 f-stops) is almost opaque to the eye and costs considerably more than an ND 4× filter. A ten-stop ND filter would be ND 3.0.

-- Olaf

I sit corrected.
My memory was faulty on the details. But indeed confusing an ND4x with an ND4 makes a big difference. And when a filter is listed as having a "grade" of 4.0, does this mean exposure factor or log-10 of the exposure factor? Some manufacturers' literature (I forget which, since it's been a while) doesn't make it clear which system they are using.

After a long search I was able to get a ND4.0 gel filter. You virtually cannot look through it. It is a 13stop ND filter. Sofar the only ND4.0's I have found were Kodak gels. They seem to have become almost extinct (for 77mm & 95mm almost for sure).

www.Heliopan.de at Munich, Germany, has different ND filters. If you can't find your filter please ask via email.
Filters for Digital Photography: http://www.heliopan.de/Heliopan-Filters.pdf

Thx!

I know, I got my 95mm ND3.0 slim screw-in from them.