deflate_compress: tweak level 4 parameters
deflate_compress: tweak level 4 parameters

Use a lower max_search_depth but a higher nice_match_length.  This seems
to turn out a bit better, on average.  This is consistent with what the
other compression levels do; level 4 was the only one that had
nice_match_length <= max_search_depth.

ebiggers/libdeflate,ebiggers/libdeflate,ebiggers/libdeflate