When I write SUM(Sheet1:Sheet4!B4:B6), that works.

But if I change the function name to SUMPRODUCT, I get a #REF error. Copied
from the Formula Bar:

=SUMPRODUCT(Sheet1:Sheet4!B4:B6)

And if I use that form of range in a subexpression, I get a #NAME error.
Copied from the Formula Bar:

=SUMPRODUCT(--(sheet1:Sheet4!B4:B6<=20))

Note that Sheet1 is lowercased by Excel. All my changes were made by
successive minimum editing of the original SUM formula.

Is there some way to make this kind of range work with SUMPRODUCT, short of
the obvious, namely breaking this into multiple subexpressions?