I will suggest you to use combination fo LookupRows, RowCount and Mod to count the orders made by this customer, and verify if the number of orders leaves any remainder when calculated by 4:
%%[
VAR @customerid, @orderrows, @rowCount, @modulo
SET @customerid = AttributeValue("SubscriberKey") /* value from attribute or DE column in send context */
SET @orderrows = LookupRows("Orders","customerid", @customerid)
SET @rowCount = rowcount(@orderrows)
SET @modulo = mod(@rowCount, 4)
IF (@rowCount > 0 AND @modulo == 0) THEN
ContentBlockByKey("myContentBlock")
ENDIF
]%%
Then use ContentBlockByKey to fetch your content block, if the statement evaluates to true.