chmod 4755 中的4是啥意思

很多中文的文章都不愿意说清楚。或者只说后面三位。然后稍微提一下第一位。

http://en.wikipedia.org/wiki/Setuid

还是上述文章写得清晰：

The setuid and setgid bits are normally set with the command chmod by setting the high-order octet to 4 (for setuid) or 2 (for setgid). `chmod 6711` will set the setuid and setgid bit (6) make the file read/write/executable for the owner (7) and executable by the group (first 1) and others (second 1). All chmod flags are octal, and the least significant bit of the high-order octal digit is used for a special mode known as the sticky bit.