vierpas in Dutch), a decorative element consisting of a symmetrical shape which forms the overall outline of four partially overlapping circles of the same diameter. It is found in art, architecture, heraldry, and traditional Christian symbolism. The word quatrefoil means 'four leaves', referring specifically to a four-leaved clover, but applies in general to four-lobed shapes in various contexts. (from Wikipedia)
Another interesting detail from the Dutch Wikipedia site about the church is that they say that this quatrefoil probably served as a 'hagioscope' or 'squint', an architectural term for a small opening or tunnel at seated eye-level through the wall of a church, that enables a view for one or more worshippers in side-chapels, chantry chapels, or other parts of the church from which the high altar was not visible.
If you're interested check the link for hagioscope for more facts.