Re: The third pillar: zakaah (poor due, charity tax)
Things in Islam depend on intention. I do not know how 401K works, but certainly, you must be knowing, how much you contributed. Thus, you should pay on money that you contributed and expect that you will get at least what you contributed back. Thus, over last many years, you have contributed X dollars, you pay zakat on that X dollars. This year you contributed Y dollars, you pay zakat next year on increased amount, that is zakat on X + Y.
You should know that zakat become due on a thing you already paid zakat previous year. Thus, you will have to pay zakat on same asset (wealth) year after year, as long as you have that asset (or wealth).
Second way is: You can make intention that what you are contributing is in the name of Allah, to unknown. Since you do not know what or if you will get anything, make intention that IF you will get something you will pay zakat on that, then probably with such intention, zakat would be only due when you will receive anything 'if anything' in the year when you will receive anything (in this way, Zakat would be due only the year you received anything and then later years if you are left with anything).
As for the example given by Lusi:
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For example if you loan your friend $10,000, will you pay zakat on it; of course not because you don’t have that money with you but your friend will.
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What I know, a person will have to pay zakat on that $10,000 too unless the loan is Qarz-e-Husna where the loan is given with intention that, if friend can repay and repay it back, JazakAllah and if friend cannot then loan is feeSabililah (forgiven in the name of Allah). If intention is that, one will take the loan back, whatever maybe, then zakat is due (as ownership of the money is not given and thus it is yours with all obligations on you). Friend will not be paying zakat on that money and if he does not pay back, still zakat will be due on you every year (until you forgive that loan amount, making it Qarz-e-Husna, or die).
Friend will never going to pay zakat on that amount because one do not pay zakat on borrowed money as one is not owner of borrowed money.