Understand the relationship between kinetic energy and frequency.

According to the photoelectric effect, the maximum kinetic energy ($E_k$) of emitted electrons is given by the equation $E_k = hf - \phi$, where $h$ is Planck's constant, $f$ is the frequency of incident radiation, and $\phi$ is the work function. The work function is also defined as $\phi = hf_0$, where $f_0$ is the threshold frequency.

The threshold frequency ($f_0$) is the minimum frequency of incident radiation required to eject electrons, meaning at this frequency, the kinetic energy of the emitted electrons is zero ($E_k = 0$). On the graph, this corresponds to the x-intercept of the line.

Locate the point where the line intersects the x-axis ($E_k \times 10^{-19} \text{ J} = 0$).
The x-axis represents frequency $f \times 10^{14} \text{ Hz}$.
The line crosses the x-axis exactly at the mark labeled '4'.

Therefore, the threshold frequency $f_0$ is:

The threshold frequency $f_0$ of the radiation is $\boxed{\text{4} \times 10^{14} \text{ Hz}}$.