the four ball test was originally developed by Shell as a simple screening test as a indication of the anti-wear behaviour of a lubricant formulation in a laboratory setting. it was regarded as a suitable test by other lubricant formulators and subsequently, after due screening, it was added to the portfolio of tests of ASTM, DIN and other standardising bodies.
It can discern quite well between oils and greases with or without EP additive and between the effects of various amounts or types of EP additive in a grease or oil and therefore is a often used test during development.
however, the conditions of the test are such that the test is not very useful in engine oil testing, since those do not contain EP additives and the conditions envisaged in the test are such that they never occur during engine lubrication.