If $$z_1, z_2$$ and $$z_3, z_4$$ are 2 pairs of complex conjugate numbers, then $$\arg\left(\frac{z_1}{z_4}\right) + \arg\left(\frac{z_2}{z_3}\right)$$ equals:

Given that $$z_1$$ and $$z_2$$ are complex conjugates, and $$z_3$$ and $$z_4$$ are complex conjugates, we can express this as $$z_2 = \bar{z_1}$$ and $$z_4 = \bar{z_3}$$. We need to find $$\arg\left(\frac{z_1}{z_4}\right) + \arg\left(\frac{z_2}{z_3}\right)$$.

Substituting the conjugates, the expression becomes $$\arg\left(\frac{z_1}{\bar{z_3}}\right) + \arg\left(\frac{\bar{z_1}}{z_3}\right)$$.

Recall the property of arguments: $$\arg\left(\frac{a}{b}\right) = \arg(a) - \arg(b)$$, modulo $$2\pi$$. Also, for any complex number $$z$$, $$\arg(\bar{z}) = -\arg(z)$$, modulo $$2\pi$$. Applying these properties:

First term: $$\arg\left(\frac{z_1}{\bar{z_3}}\right) = \arg(z_1) - \arg(\bar{z_3}) = \arg(z_1) - (-\arg(z_3)) = \arg(z_1) + \arg(z_3)$$.

Second term: $$\arg\left(\frac{\bar{z_1}}{z_3}\right) = \arg(\bar{z_1}) - \arg(z_3) = -\arg(z_1) - \arg(z_3)$$.

Adding both terms: $$(\arg(z_1) + \arg(z_3)) + (-\arg(z_1) - \arg(z_3)) = \arg(z_1) + \arg(z_3) - \arg(z_1) - \arg(z_3) = 0$$.

Thus, the sum simplifies to 0, modulo $$2\pi$$. Since the principal argument lies in $$(-\pi, \pi]$$, the sum is exactly 0.

Hence, the correct answer is Option A.