There are 5 answers for this question but it says 4.
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Two of the answers were posted at 12:19:28, in the same second. This probably lead to a race condition when updating the answer count.
Since this happens from time to time with all the the different counters, there are scripts in place that run daily to correct such invalid counts. Probably until tomorrow the count will be corrected.
Of course they could use transactions in the database to avoid such problems, but obviously this isn't done in Stack Overflow.
One of the answers is deleted, that's why (I believe the post count is denormalized, so that explains why it isn't immediately updated when a answer is deleted).