There are plenty mills shown on this board with around 1Kw output. Take one of those, double the diameter of the magnetic rotor and get 4x the output. This is assuming you make no other changes to the alternator.
Blades will be your biggest problem, I made a 4Kw down wind mill two years and that was a tough mill to match blades. Of all the attributes of mill building, blades have never been subject to much new design, taking aircraft profiles are scaling them for wind usage is simply incorrect. Start with low reynolds numbers, and then design. Blades that stall in higher wind speeds on dual rotor mills demonstrates this perfectly.
Most home brew mills shown in here, have pretty efficient alternators. But blades leaves em all down and mitigates our gain from good alternator design. Albeit at a magentic higher € cost, but worth it.
A word to the wise though, machines over 2Kw get very angry and are spectular when they let go. Build it 'well' and build it with good engineering practise, hose clamps, duct tape and cheap (or worse no) bearings will bite you!....and its a waste of good magnets :]
