I really wish they'd make an 400/F4 PF lens.
I agree, it is very versatile, you can put a TE 1.4 on it ;-) I say is because that it what I can do with my Canon 400mm DO II. (Diffractive Optics is Canons version of Fresnel optics)
I would prefer a 400 PF F4 over a 500mm Pf F5.6 or a 600mm F5.6 PF. How ever an F4 lens is little heavier and larger than a 500mm pf just so you all know