Interesting question.
U.S. Immigration Policy on Temporary Admissions updated 25 January, 2007, pg 22, K-1 Visa durationI thought for sure it was 6 months but I can find no confirmation of this other than a note on the VJ guides that states the 6 months period. I was very surprised that this does not appear to be addressed on the Dept of State or the American Embassy websites (I only checked the American Embassy in Moscow. A check of the corresponding American Embassy in Cambodia website might provide some information on this). Most likely I am missing something somewhere but I cannot find it. I did find the above document as a report to Congress that states 4 months, however, I would certainly defer to someone who has been through the entire process and can look at their visa for the date of issue v. date of expiration. The folks who have answered above may have this evidence at hand and may very well know more about this than I do. I was also unable to locate an answer after searching U.S. Code.
Anyone who is familiar with my posts about such matters knows that I may not know the answer myself but I will go to great lengths to find the answer. Presented above for your review is the only answer I could find after about 1 hour of searching. Please note that the document refers specifically to the K-1 Visa and not the application for the Visa.
It may be a good idea to contact USCIS with this question or try and contact the US Dept of State. They are very difficult to get a straight answer out of and chances are that they will refer you to USCIS anyway.
It sounds like something you really need to get a solid answer on. I am hopeful that maybe one of the previous posters can verify this either with personal experience or a statutory reference indicating 4 or 6 months. My fiancee will not be here for another month and only then can I look at her documents and definitively answer this based on the Embassy in Moscow.
Good luck.