Part 4 of the EN standard is quite unknown. It uses an edge to meaure the unsharpness due to the focal spot. The physics is similar to the . The detector is a film.
The apparatus which provides the edge is a cylinder or tube made of steel with 50mm to 100mm diameter where a 1mm thick sheet of lead is bended around to provide the edge:

The evaluation is done with a magnifying glass with a mm scale. The length from the point where the density is reduced to 95% of max. signal is the beginning of the focal spot until the density is reduced to 5%:

The length is between the 95% and 5% density values - both values are quite unusual in NDT standards due to the problem of scatter radiation and density of the material.
The measurement has to be repeated and the apparatus turned by 90 degrees for the second exposure.

The film only specification and the problem to get a planar surface with the lead may be the reason that part 4 of the EN standard is not used very often for NDT focal spot measurements.