HOW LONG WOULD IT TAKE TO ASSAY 4 METRIC TONS OF GOLD DEPOSITED IN A BULLION BANK?

I take this to mean you are looking to electrically plate out the
metal from the bank. this is a problem that was discussed in my
chemistry class and the answer is directly related to the amperage
applied. the material you are replacing the gold with dictates the
necessary voltage. Suffice it to say that the quantity of both time
and electricity would be astronomical, with the electricty, billed at
8 cents per kilowatt hour likely costing more than the gold, and the
amount of time, assumign you are using somewhere around 2000 watts
(the maximum for a standard 120v cuircit) many many lifetimes.

In practice, one would generally assay some random sample of bars,
rather than going through the whole stash an atom at a time.  How long
this would take would depend on the number of bars sampled (more bars
= higher confidence in the results), and the methods used.  The
density of a bar could be measured fairly quickly and would identify
any bars that were very impure (gold plated lead), though it would not
be as good at telling 90% gold from 99% gold.  Other assaying methods
would be more accurate, but also more time consuming.