I'll only speak for the Pentax K5 as I don't own a Nikon, Canon or Olympus.
The K5 is probably the best choice for landscapes and street candids as it's the most compact of the bunch in terms of body and lenses (overall). Also, in term of build it's very sturdy and weather resistant (WR). Combine it with some WR lenses and you have a fantastic hiking, all weather, combo! (WR lenses: 16-50 f/2.8, 50-135 f/2.8, 55 f/1.4, 100 macro f/2.8, 60-250 f/4, 300 f/4, 18-55, 55-200, etc.)
If she's into primes, it's probably the best choice as well, notably for candids because the small form factor make it a very discreet package. Very small pancake (or almost) lenses are also unique to Pentax: 15 f/4, 21 f/3.2, 40 f/2.8, 43 f/1.9, 70 f/2.4.
Ergonomics are also arguably the best with Pentax too.
However, where Pentax is not great is for long lenses. There is nothing longer than 300mm unless you go with Sigma. I suspect autofocus is probably better with Nikon/Canon (not sure about Oly though).
I do a lot of landscape, nature, macro and portraits shots and I think for that purpose the K5 is the best camera. I just can't see myself carrying bricks in my backpack on hikes.
Finally, the K5 has the highest DR of the cameras in your list. In fact, it's got the highest DR of any APS-C and fullframe camera in the market today. At ISO 80, the DR is just incredible (check dxomark.com for a comparison). The Nikon is very close as they share the same sensor. However, the K5 goes down to ISO 80 which is a very nice addition.
You can check my blog entry [url="http://capturedbyflo.blogspot.com/2011/05/post-processing-magic.html"]here[/url] as an example of the recovery power you can get with almost no noise.
The K5 is probably the best choice for landscapes and street candids as it's the most compact of the bunch in terms of body and lenses (overall). Also, in term of build it's very sturdy and weather resistant (WR). Combine it with some WR lenses and you have a fantastic hiking, all weather, combo! (WR lenses: 16-50 f/2.8, 50-135 f/2.8, 55 f/1.4, 100 macro f/2.8, 60-250 f/4, 300 f/4, 18-55, 55-200, etc.)
If she's into primes, it's probably the best choice as well, notably for candids because the small form factor make it a very discreet package. Very small pancake (or almost) lenses are also unique to Pentax: 15 f/4, 21 f/3.2, 40 f/2.8, 43 f/1.9, 70 f/2.4.
Ergonomics are also arguably the best with Pentax too.
However, where Pentax is not great is for long lenses. There is nothing longer than 300mm unless you go with Sigma. I suspect autofocus is probably better with Nikon/Canon (not sure about Oly though).
I do a lot of landscape, nature, macro and portraits shots and I think for that purpose the K5 is the best camera. I just can't see myself carrying bricks in my backpack on hikes.
Finally, the K5 has the highest DR of the cameras in your list. In fact, it's got the highest DR of any APS-C and fullframe camera in the market today. At ISO 80, the DR is just incredible (check dxomark.com for a comparison). The Nikon is very close as they share the same sensor. However, the K5 goes down to ISO 80 which is a very nice addition.
You can check my blog entry [url="http://capturedbyflo.blogspot.com/2011/05/post-processing-magic.html"]here[/url] as an example of the recovery power you can get with almost no noise.