Again nothing much in it - surveyors original answer also applies to the above instance.
I don't fully uderstand the above:
If you mean you can get 1600C8 for the same price of 1866C9 to match your existing 4GB of 1600C8 making 12Gb - then i would go for the 1600C8.
The 1600C8 has tighter timings and it's always advisible to get matching sets of memory where possible to help rule out possible compatibilty issues.
However, you could possibly run the 1866MHz at 1600MHz with tighter timings - but this may not be possible (the timings that is). The reality is you may have to slacken the timings of your existing memory - which wouldn't be a problem if that was your want. Swings and round-abouts...