Fund withdrawals from such plans triggers early withdrawal penalty (in case the funds are withdrawn before one reaches the retirement age) and taxes. Hence, Zakat on 401K, IRA and other retirement plans will only be due when the funds are received. At that time the person must pay the Zakat of all the past years, and not just one year, i.e. ever since he became the owner of Nisab and ever since this money reached that limit. This will require a lot of effort in keeping track of each year's Zakat-able amount until the funds have been withdrawn. Hence, it is advised by the scholars to pay Zakat on the 401K, IRA plans, etc. plans yearly, to avoid miscalculations after several years of accumulation.