I've got two ND filters that I use. A 10 Stop Formatt Hitech Filter and a Lightcraft Fader ND mkII. I can't imagine shooting landscapes without them. The ability to shoot 30 second exposures outside in the middle of the day is sooo awesome. ND4 is quite limiting as it only cuts out 2 stops of light. So if the shutter speed is at 1/250th it would affect it by 2 stops making it 1/60th, which really isn't that much difference. But it all depends on what time of the day you are shooting at.
Early morning and late afternoon is the best time to shoot as there isn't much light around anyway.