dependant subquery processed despite Extra Impossible WHERE noticed after reading const tables
Look at the Handler counts to see if it is actually touching any rows:  http://mysql.rjweb.org/doc.php/index_cookbook_mysql#handler_counts

And use EXPLAIN FORMAT=JSON SELECT ... to get more details.

Anyway, since it is a DEPENDENT subquery, it will be asked to run as often as needed.  But that is zero times.

Also, EXPLAIN, is not perfect.  The subquery did not get 4 rows; EXPLAIN is estimating that it will touch 4 rows each time it the subquery is invoked.  Again, 0*4 = 0.