There are not that many options for Pentax users who wish to go to 400mm or beyond. Sigma do have some lenses in their range, but most others tend to be Canon or Nikon only.
If one can be found, the Tokina 80-400mm AF lens exists in two versions. MkII looks identical to MkI bu8t adds a tripod collar and offers improved performance. These are both heavy, well made lenses and could be ideal if we need to get to that magical 400mm at a reasonable cost.