def plot_histograms(ax, prng, nb_samples=10000):
    """Plot 4 histograms and a text annotation.
    """
    params = (10, 10), (4, 12), (50, 12), (6, 55)
    for a, b in params:
        values = prng.beta(a, b, size=nb_samples)
        ax.hist(values, histtype='stepfilled', bins=30, alpha=0.8, density=True
            )
    ax.annotate('Annotation', xy=(0.25, 4.25), xycoords='data', xytext=(0.9,
        0.9), textcoords='axes fraction', va='top', ha='right', bbox=dict(
        boxstyle='round', alpha=0.2), arrowprops=dict(arrowstyle='->',
        connectionstyle='angle,angleA=-95,angleB=35,rad=10'))
    return ax
