dB is a log10() kind of equation*. So, doubling the power (not the apparent noise, the actual acoustic power) is about 3 (10*log10(2) = 3.0103). Tripling the power is about 4.77, and quadrupling the power is about 6.02.
So, add about 3 for the second fan, and add another 1.7 for the third fan. Four the fourth fan, just add another 1.3dB.
But, that's the power measured by the ability to do work. For apparent sound level, an additional 10dB is about twice as loud to the human ear.
* The dB is a logarithmic unit expressing the RATIO of two powers. It is defined as:
dB = 10 * log10(P2/P1).
When measuring pure amplitude, you need to modify things a bit, since the proportion is based upon the square of the two amplitudes. log(x^2) = 2*log(x). So,
dB = 20 * log10(A2/A1)
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