OK, what are the lighting conditions? Sunny day? Dark room? Moonlit backyard? Using 12 1000ws monolights?
Sunny trying to test it out, as you can tell first time trying it out
If you "meter" your shot on a sunny day, the ND4 filter allows you to change the exposure by 4 stops.
Suppose we use the Sunny 16 rule (since you said it was sunny).
A "baseline" exposure would be ISO 100, f/16, and 1/100th (although that's just a baseline... you actually get to use any "equivalent" exposure using the exposure triangle. If you're not familiar with the exposure triangle, you might want to pick up a copy of "Understanding Exposure" by Bryan Peterson.) We know this without even using a light meter since the sun pumps out a _very_ consistent amount of light in the middle of a sunny day. So as long as we're not talking about early morning or late evening "golden hour" light or it's not a cloudy day, the sunny 16 exposure will be correct.
From that baseline, you may now slow down your shutter speed by four stops. Each "stop" of the ND blocks half the light. That means you're getting 1/2 of 1/2 of 1/2 of 1/2 of the original light (1/16th). You may now slow the shutter down by that much to compensate for it.
1/100th x 2 = 1/50th. 1/50th x 2 = 1/25th. 1/25th x 2 = 1/13th (I rounded that last one).
You didn't use f/16, however... you used f/11. f/11 is one "full" stop larger which means it let's in half the light. So now you have to back off the shutter by a full stop and return to 1/25th.
Your 8 second exposure was 32 times longer than needed... hence you got an all white image.