This is the code: OBKRUOXOGHULBSOLIFBBWFLRVQQPRNGKSSOTWTQSJQSSEKZZWATJKLUDIAWINFBNYPVTTMZFPKWGDKZXTJCDIGKUHUAUEKCAR (I believe)
First, we were told the answers to the first 3 contain hints to the 4th. In november 2010, We got a hint saying that "NYPVTT", the 64th-69th letters in part four, become "BERLIN" after decryption.
We also later got a clue in November 2014, saying the letters "MZFPK", the 70th-74th letters in part four, become "CLOCK" after decryption.
This may be a direct reference to the Berlin Clock, and the creator further stated that in order to solve section 4, "You'd better delve into that particular clock," but added, "There are several really interesting clocks in Berlin."
That's about it. Lots of this info was found on Wikipedia and ill of course link it if you want to read up on it.
I don't really have anything to add except to direct to an interesting YouTube video posted by Klaus Schmeh. It was a "Kryptos Workshop" 2015-10-25. Here is the link: Kryptos Workshop. It is very interesting.
What I got out of it was mostly that K4 involves masking. There was much talk about duress codes, and that Ed Scheid said he did not decode K4. Also it seems, like we already know, K1-K3 contribute to K4 in some way.
Since our last challenge I've been thinking a lot about encoding systems. I was thinking about binary - in particular, Grey Code / reflected binary. Thinking of the water / reflecting pool.