^start of line
.{4}any character (except for line terminators), repeated exactly 4 times
\Kresets the starting point of the reported match. Any previously consumed characters are no longer included in the final match
.+any character (except for line terminators), repeated at least once
g modifier: global. Finds all matches instead of stopping after the first
m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string