are you calculating the 4.7m/s at 30m from the NOABL database?

If so I would strongly suggest that you get some real data from a logging anemometer on the site for at least 3-4 months over the winter. They are available for about £100 which is a pretty small fraction of what you are about to spend on a 6kW turbine. You'd need to rig it up on a 30m section of scaffold pole at the location you're planning to put the turbine.

As Stuart says, 4.7m/s is really quite low to consider suitable for a turbine site. But the NOABL figures are a mathematcally modelled extrapolation of a few real readings averaged over a large area and are not more than a rough guide.

ted