The utfmb4 character set has these characteristics:
Supports BMP and supplementary characters.
Requires a maximum of four bytes per multibyte character.
utf8mb4 contrasts with the utf8mb3 character set, which supports only BMP characters and uses a maximum of three bytes per character:
For a BMP character, utf8mb4 and utf8mb3 have identical storage characteristics: same code values, same encoding, same length.
For a supplementary character, utf8mb4 requires four bytes to store it, whereas utf8mb3 cannot store the character at all. When converting utf8mb3 columns to utf8mb4, you need not worry about converting supplementary characters because there are none.
utf8mb4 is a superset of utf8mb3, so for an operation such as the following concatenation, the result has character set utf8mb4 and the collation of utf8mb4_col:
SELECT CONCAT(utf8mb3_col, utf8mb4_col);
Similarly, the following comparison in the WHERE clause works according to the collation of utf8mb4_col:
SELECT * FROM utf8mb3_tbl, utf8mb4_tbl WHERE utf8mb3_tbl.utf8mb3_col = utf8mb4_tbl.utf8mb4_col;
For information about data type storage as it relates to multibyte character sets, see String Type Storage Requirements.