Which lens next?

So I currently own, Sigma 10-20/24-70 f2.8/70-200 f2.8/pentax 50 f1.4.And the Sigma 105mm macro. Now need something for the 200-400mm range. I see sigma have a new 120-400 O?S lens in pentax mount, would this perform well on the K20d and would you need to turn of shake reduction in cam?

That sigma ought to perform fine on a K20d. It is (I think) an HSM lens so you will need to make sure you have the right firmware (or at least did for the K10d - maybe the K20 supports HSM right out of the box. In terms of O/S I imagine you would need to turn one of the systems off, otherwise I guess they'll confuse one another. The previous incarnation of the lens (less O/S) is a good lens in my view, although it's a big old lump to deal with (a tripod helps) and quite long when out at 400mm. Better performance if you can stop the aperture down a bit so comes into it's own more in good light - ideal just as we're heading into summer. If you want to get out to 400mm or beyond then (in terms of new lenses at least) the Sigma range of lenses or a teleconverter on your existing 70-200 are pretty much your only choices I think. You could also maybe look at the DA* 300mm which is obviously not 400mm but it is very sharp.

Regards