A 400mm lens for a sunset would be a little unusual but not by any means out of the question, it is all down to the scene and what you want in the shot.
The problem with sunsets is the large dynamic range. The sky is likely to be very bright and the foreground dark and the camera sensor cannot correctly record the darkest and lightest areas.
There are a number of solutions
Bracket exposures so you have a range of shots some that cover the full range of light to dark areas and then combine them in pp via editing packages or dedicated HDR software. HDR often has some running for the hills, screaming or reaching for 12 bore, but done well it is a useful technique. In some cases it is not essential but a tripod is very handy when bracketing.
Take one shot that is expose for the lightest area (or the darkest area) and then try to pull some detail from the under exposed (or over exposed) areas of the image. The drawback with this is pulling detail out of very dark areas can increase the apparent noise in the image and if the over exposed areas are really blown then no amount of work in pp is going to bring them background
Use graduated ND filters. These help reduce the dynamic range that the sensor sees and in many cases allow you to get a shot that otherwise would not have been possible.
As you are likely to want a large depth of field, a small aperture and consequently a slow shutter speed is likely, so a tripod would be very useful. Then its down to, unless you know an area well, plenty of planning, a eye on the forecast, getting there with time to spare, the patience of a saint and the ability to accept you could, after all the best preparation, be disappointed.
Dave
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