This error, which is not documented anywhere I can find online, comes from having two commas in a row with nothing between them inside the array.
In my case, this actually appeared on line 42 of the file, not line 4 as indicated by the error message, which sounds like a bug in the compiler which identifies the first item in the array instead of the actual location of the “empty array element”.
NOTE: In PHP 7.2.15+, 7.3.2+, and 7.4.0+, the error message has been changed to report the line number of the previous valid element instead of the line number of the beginning of the array. While this may still be off by one or more lines, it is usually close enough to the problem to make it much easier to find.