Here is something similar

http://www.kan.org/tips/excel_sumproduct_advanced3.php

Not entirely sure where your data reside in the sheet, but you could add a
helper row and then use the function MOD(helper cell, 4)=0 as one of the
conditions in your SUMPRODUCT
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Tips for Excel, Word, PowerPoint and Other Applications
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"Darius" wrote:

I have a worksheet that has over 2000 rows and every 4th row I have a number
that I am trying to sum up. Currently I understand that I can not have more
then 30 variables in a function so I take 30 rows at a time then sum up the
results and adding them up. Here is what I am using now as a formula:
=SUM(C4,C8,C12,C16,C20,C24,C47,C51,C55,C59,C63,C67 ,C28,C32,C37,C71,C75,C79,C83,C87,C93,C97,C101,C105 ,C109,C113,C117,C121,C125,C129)
I take that result and added to the rest for ever 30 rows

The worksheet is getting bigger by the day and I need something that adds
every 4th row no matter how many rows I have. All the help will be greatly
appreciated. Thank you.