Hi Matt,
Some others are:
SMC Pentax-FA* 400mm F5.6 ED [IF] Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database Sigma APO 135-400mm F4.5-5.6 DG Lens Reviews - Sigma Lenses - Pentax Lens Review Database Tokina AT-X 80-400mm F1: 4.5-5.6 Lens Reviews - Tokina Lenses - Pentax Lens Review Database
These are all discontinued, and only available used. The recent market for the FA* 400 f5.6 puts them in the impractically pricey range (close to $2000 USD), IMO. From what I've seen they're very good, but not in the top tier of Pentax * class ultra tele lenses, IMO. Personally, I'd rather use an FA* 300 f4.5 + Tamron F 1.4x AF PZ MC4 TC to get 420mm f6.3, and in fact, that's exactly what I do. . .
I have the Tokina 80-400 f4.5-5.6 AT-X, and it's a competent lens with a very wide zoom range, but a bit soft at 400mm wide open. With stopping down a bit and some judicious sharpening in PP, I can get reasonably good results though.
When both the Sigma 135-400 and Tokina 80-400 were both available new, there was an on-going debate with Pentax users of each about which was better, and I don't think it was ever really resolved. User reviewers here seem to prefer the Tokina if you compare the ratings. I always considered them pretty equal, but have never shot the Sigma. From what I've seen, the current Sigma 120-400 is a better lens, and not as compromised at 400mm -- but it's a lot more expensive.
From what I've seen over the years, for the
best bang for the buck in 400mm lenses in K mount, I'd say the Sigma 400 f5.6 APO is probably easily in a class by itself if you can find one.
Scott