Just to show TIMTOWTDI, I add this:

#!/usr/bin/perl -w

use strict;

$_ = '1234567890';

s/.(?!.{0,3}$)/x/g;

print;
[download]

Update: Looks like this solution lost bigtime in the benchmarks, which is no surprise. It replaces each character one by one, and each time checks if the next 1-4 characters are among the last four. Hardly effective... It is a good example on the fact that regexps are costly, and not always the best solution. :)

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You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.