Error message: Warning: Cannot modify header information

I just moved my site from one host to another, by manually moving the files and MySQL server from the first host to the second. Now when I enter the site from the new host, I get the following error message:

Warning: Cannot modify header information – headers already sent by (output started at /home/iosjail1/public_html/index.php:4) in /home/iosjail1/public_html/wp-includes/pluggable.php on line 897

Here is line 893-899

if ( !empty( $headers ) ) {

		foreach( (array) $headers as $name => $content ) {

			$phpmailer->AddCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );

		}

I have already edited wp-config so the mysql server details are correct. Hope someone is willing to help. Thank you 🙂