What do find -mtime -4 and find -mtime +4 do? I cannot understand the examples given in the man page.
As per man page:
-mtime n
File’s data was last modified n*24 hours ago. See the comments
for -atime to understand how rounding affects the interpretation
of file modification times.
The argument to -mtime is interpreted as the number of whole days in the age of the file. -mtime +n means strictly greater than, -mtime -n means strictly less than.
So in your case,
-4 meaning less than 4 days.
+4 meaning more than 4 days i.e. 4*24 hours.