How to convert a single MySql table to utf8mb4 (from latin1)
Going by what you have to do within a PHP-Webserver environment to use utf8 / utf8mb4, you have up to 5 or more construction places. And you covered 2 of them 


collation of table and column(s)
overwork existing string-data in said columns 


leaves:


make sure your application / tool uses the correct collation as well
make sure you communicate with the database using the correct collation
set the frontend of your application / tool to handle said collation 


As JAVA is well prepared I guess its the database connection ...