Hi!
I do not understand what your problem is. UTF8 character may be 4 bytes long by standart. Postgresql can store this characters. Mysql - for history reasons - treat utf8 as at most 3 byte characters and is not fully unicode compatible. In mysql 5.5 was added utf8mb4 charset which is full unicode compatible and can store 4 byte characters ( https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html ). Discarding or correct storing 4-byte symbols is not vulnerabilities, just historical error in mysql.
PS: given link is broken due outdated https cert.