There are numerous options here

=SUM(E28:H28)/4
and use a Custom Number Format to mask 0

Or double evaluate the SUM and return a Formula Null (text string)

=IF(SUM(E28:H28)=0,"",SUM(E28:H28)/4)

which would also be written as

=IF(SUM(E28:H28),SUM(E28:H28)/4,"")

Note: in using SUM there is no need for the + operator (this is one of the reasons it is used - ie to avoid explicit coercion).
Where the ranges are contiguous use a contiguous reference else delimit ranges with comma