4.6 billion is the minimum price the FCC is willing to actually sell for, as in if nobody bids as much as 4.6 the FCC won't actually sell. So by saying to the FCC we'll bid 4.6 they assure the FCC they'll get the price they wanted.
In exchange for this assurance Google got 2 out of 4 condition for free access to the frequency.
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4.6 billion is the minimum price the FCC is willing to actually sell for, as in if nobody bids as much as 4.6 the FCC won't actually sell. So by saying to the FCC we'll bid 4.6 they assure the FCC they'll get the price they wanted. In exchange for this assurance Google got 2 out of 4 condition for free access to the frequency.