sub readNLines {
    my $lines_to_read = shift or die '....';
    my @lines_read;
    while(<>) {
        push( @lines_read, $_ );
        last if @lines_read == $lines_to_read;
    }
    if ( @lines_read != $lines_to_read ) {
        # error condition
    }
    return @lines_read;
}

while (@lines = readNLines(4)) { do_stuff() }