Second Pic is my engine that was fully rebuilt about 2 years ago....still going strong ;).
The head you see on it is a 3K bigport head, you wont get much bigger ports than that in a K engine head....those are just about the limit. I found that building my own engine, gave me alot more insight into what goes where and how to make more power....When i joined the club (just before i started the rebuild) i didnt know much about k engines.....now that i've rebuilt and worked on one many times, i can honestly tell you i don't regret any bit of spending money on a K engine (i'm still doing it lol).
Felix is right, you wont really feel much difference between the 4K and 5K....torque and a bit more get up and go. And anyone that says you can't get a 5K to rev cause they are a "van engine" needs to have a look at some of the 5Ks on here....lol