The K-4 has a better engine that's also geared for higher alt work (it's way better at 25k than the G14 is!) but did not have as many weapons options as the G-14 did. The G-14 was fairly close to the K-4 under 16k, but a bit slower (this means it also climbed a little slower, and accelerated a little slower) -- but it still is close to the K-4.
Take the K-4 for speed and climb. Take the G14 for weapons options and turn rate. That being said, you can still turn with the K-4 and you can still climb and run with the G14. One's just better suited than the other.