Use:

import matplotlib.pyplot as plt

plt.gcf().subplots_adjust(bottom=0.15)

# alternate option without .gcf
plt.subplots_adjust(bottom=0.15)

to make room for the label, where plt.gcf() means get the current figure. plt.gca(), which gets the current Axes, can also be used.

Edit:

Since I gave the answer, matplotlib has added the plt.tight_layout() function.

See matplotlib Tutorials: Tight Layout Guide

So I suggest using it:

fig, axes = plt.subplots(ncols=2, nrows=2, figsize=(8, 6))
axes = axes.flatten()

for ax in axes:
    ax.set_ylabel(r'$\ln\left(\frac{x_a-x_b}{x_a-x_c}\right)$')
    ax.set_xlabel(r'$\ln\left(\frac{x_a-x_d}{x_a-x_e}\right)$')

plt.tight_layout()
plt.show()

enter image description here