The superficial degree of divergence for a diagram is defined as the power of $k$ in the nominator minus the power of $k$ in the denominator. It is written to be equal to $4\times$ (number of loops)$ - $(number of internal Fermion lines)$ - 2 \times $(number of internal Boson lines).
Why is it 4 and not 3? When you do a 4-dimensional integral in spherical coordinates, you get a factor of $k^3$ from your volume element.
In particular, when you calculate the QED vertex, for instance, you get:
$$ \int_{\mathbb{R}^4} \frac{d^4k}{(2\pi)^4} \frac{\gamma^\rho (\not\!{p_1}-\not\!{k}+m)\gamma^\mu(-\not\!{p_2}-\not\!{k}+m)\gamma_\rho}{k^2\left[\left(p_1-k\right)^2-m^2\right]\left[\left(p_2+k\right)^2-m^2\right]} \overset{k\to\infty}{\propto} \int_{\mathbb{R}^4} \frac{d^4k\, k^2}{k^6} \overset{\text{spherical}}{\underset{\text{coordinates}}{\propto}} \int_0^\infty\frac{dk}{k} $$
Thus I would say that the superficial degree of divergence of this diagram is $3\times1-2-2=-1$ and not $0$.
What am I missing?