"Employment" means services performed after December 31, 1971, by an individual in the employ of a religious, charitable, educational, or other organization which is excluded from the term "employment", as defined in the "Federal Unemployment Tax Act" solely by reason of section 3306 (c)(8) of that act, and which has had four or more individuals in employment for some portion of a day in each of twenty different weeks, whether or not such weeks were consecutive, within either the current or the preceding calendar year, regardless of whether they were employed at the same moment of time.