The PK380 is not a Walther -- it's cheaply-made Umarex junk stamped with the Walther name. Forget that thing.
On the other hand, I cannot praise the PPS highly enough. It's pretty easily best-of-breed among the ultra-slim, single-stack 9mm pistols, in my opinion. Build quality, reliability, and accuracy are superb. I shoot my PPS about as well as a full-size pistol out to 15 yards, and I find the recoil even with 124-grain +P Gold Dots to be very mild for the pistol's size/weight. A lot of PPS owners will tell you similar things. The PPS has the trigger a Glock wishes it has; if it doesn't start off sublime (for a striker-fired gun's trigger), it will get there with a little bit of use. I really can't think of a single negative thing to say about it.
The Shield is also a good gun, but the PPS doesn't cost as much as it used to anymore. I recommend the Shield to plenty of folks. But if you're comfortable spending a bit more, I'd go with the PPS without a second's hesitation.
Forget the PX4 SC if you want something slim. It's a very well-made gun, but it's very chunky. You can look at a Beretta Nano if you want to explore another option. Build quality and refinement are there with the PPS, but the trigger, which some like, is much longer and heavier than what I want. The ergos don't work for me that well either. Even though it's a higher-quality gun than the Shield, I would probably opt for the Shield over the Nano due to the trigger and ergos, and because the Shield is still plenty good on the quality count.