with open('filename_number.txt', 'r', encoding='utf-8') as infile:
    lines  = infile.readlines()
    processed_lines = []
    for line in lines:
        stripped_line = line.rstrip('\n\r')
        if len(stripped_line) > 4:
            new_line = int(stripped_line)
        else:
            new_line = ""
        processed_lines.append(str(new_line) + '\n')

    with open('filename_number_int.txt', 'w', encoding='utf-8') as outfile:
        outfile.writelines(processed_lines)