Warning: Cannot modify header information

when i go to my login page for my wordpress, i get this error message.

Warning: Cannot modify header information – headers already sent by (output started at /home/katelynt/public_html/index.php:4) in /home/katelynt/public_html/wordpress/wp-includes/pluggable.php on line 694

so i guess its a problem with my pluggable.php file, but that file is extremely long and the lines aren’t numbered so i have no idea how to find the problem.

can someone please help me.