uft8mb4 only means that the database is able to store all kinds of UTF-8 sequences. However it seems the strings you are trying to store is not valid UTF-8. If you are using Windows the default charset is CP1252 and not every CP1252 string is automatically a valid UTF-8 string. You can try to add
-Dfile.encoding=UTF-8
to your knime.ini (at the bottom). Also if you post your workflow we could check whether we can work around such situations automatically.
I couldn't reproduce your problem. I took your workflow, just changed the connection details in the Database Writer, used a database that has the charset set to utf8mb4 and was able to sucessfully insert all rows.
What does "SHOW FULL COLUMNS FROM apple" say about the collation for the "Tweet" column? In my case it's "utf8mb4_general_ci".
Sorry, it also works if I set the collation to "utf8mb4_unicode_ci". I have no idea what the problem could be. Can you send me the relevant part of the knime.log? It may contain more information about the actual database error.