I'm assuming that either way you go, you're going to have both film and digital bodies in that mount. If it's constant aperture you're after and you're not necessarily stuck on a constant f/4, then how about picking up a Tamron 28-75/2.8 for both digital and film Pentax bodies? On the film body, 28 would be pretty wide and if you need to go wider than 28 on the digital, you could always get a DA 12-24/4. For the longer end of things, a Sigma or Tamron 70-200/2.8 or the Sigma 100-300/4 would work on both bodies.
If you have your heart set on f/4, then there's always the FA28-70/4:
FA 28-70/4 AL
If you don't mind a MF lens, then there's the A70-210/4:
A 70-210/4
As for film bodies, you might want to go check out the film slr section of the forum. I know one more recent body that people really seem to like is the MZ/ZX 5-n:
MZ-5n / ZX-5n
HTH,
Heather