Calculating run time with O(nlog2n)

[x1222]

If the run time for the process is 0.4s for when n = 1000, what's the runtime for n = 4000;

Should it be 4000log2(4000) / 1000log2(1000) * 0.4s?

Or is it done this way: 4000 is 4x 1000, and 4 is 2^2,
so it is (4000 / 1000) * 2 greater, or 8 * 0.4s.