Index: src/wp-includes/wp-db.php
===================================================================
--- src/wp-includes/wp-db.php	(revision 31939)
+++ src/wp-includes/wp-db.php	(working copy)
@@ -2219,6 +2219,12 @@
 		foreach ( $columns as $column ) {
 			if ( ! empty( $column->Collation ) ) {
 				list( $charset ) = explode( '_', $column->Collation );
+
+				// If the current connection can't support utf8mb4 characters, let's only send 3-byte utf8 characters.
+				if ( 'utf8mb4' === $charset && ! $this->has_cap( 'utf8mb4' ) ) {
+					$charset = 'utf8';
+				}
+
 				$charsets[ strtolower( $charset ) ] = true;
 			}