K3, not K4
I've had the K3 for a while now, and I love it. Here is the complete rundown between the two, and why you should buy the K3.
Workpiece clamping: K3 is far superior in that it clamps from the front so you don't have to reach around the back of the workpiece to clamp it. When Kreg made the K3, they marketed the front clamping as a huge improvement over the K2's rear clamping. So I'm not sure why they reverted back to rear clamping for the K4. The K3 wins.
Clamping to workbench: I like the idea that the K4 has a place to use a clamp to secure the jig to a workbench. But that assumes you'll always be using the jig on the edge of a bench, table, etc. I have my K3 screwed onto a piece of 3/4" plywood about 12x16", with a hole in it to carry or hang on a wall. So when I want to use the jig, I just put that piece of plywood down on a bench, floor, or wherever. You can do the same kind of mounting with the K4, but then the clamp area on the front of the jig is not necessary and that came at the expense of moving the workpiece clamp to the rear of the jig. The K3 wins.
Dust collection: the K3 comes with the dust shroud, the K4 doesn't. I find it works EXTREMELY well. I hook up my Fein TurboII to it and I've never had chips or dust get stuck in the shroud or pocket hole guides. The K3 wins.
Repeat flip stop: the K3 comes with this and the K4 doesn't. I've never used it and don't think it is of any value. I've never done an application where I needed exact repeat hole placement, so the flip stop was never used. I also hide the pocket holes, so I don't care if one hole is 1" in from the edge of the wood, and another is in 1.1" from the edge. This shouldn't lean you either way toward K3 or K4.
Face clamp: the K3 comes with a face clamp, the K4 doesn't. This is a $25 clamp I think. You'll need one anyway, so might as well get it with the K3.
So anyway. Is the K3 worth more than the K4? It is, but that is because you're paying extra for the "included" accessories of the dust shroud, flip stop and face clamp. The value of the included accessories may be a little more or a little less than the price differences of the two units. But in my opinion, the front clamping of the K3 alone is enough to make it the winner over the K4, even at the higher price.