open(FH,"test.txt");

while ( <FH> ) {

        ## If the line matches for this pattern, I should read 4th line from current line
        if ( $_ =~/you have to read the fourth line/ ){
                $i=0;
                while ($i<4)
                {
                        $line = <FH>;
                        $i++;
                }
                print $line;
        }
        ## If the line matches for this pattern, I should read 2nd line from current line
        elsif ( $_ =~/you have to read the second line/ ){
                $i=0;
                while ($i<2)
                {
                        $line = <FH>;
                        $i++;
                }
                print $line;
        }

}