What is the meaning of 4?

The cardinal number four.

A digit in the decimal system of numbering, and also in octal, and hexadecimal.

the pitch of a tone, either high or low depending on local convention (though not as high or low as 5, if '5' is also used)

tone number 4, typically identified with light rising yang shang (陽上). May be identified with entering ru tone () if the yinyang distinction is not made.

abbreviation of for

Used after an isolated affix or clitic to indicate that it has four variant forms, dependent on the vowel harmony of the preceding stem or word.

Source: wiktionary.org