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The LOOKUP() function lets you look forward or backward in your table. So:
IF NOT ISNULL(LOOKUP(SUM([Sales]),4)) THEN 1 END
This would set a value of 1 in the current month for the given mark (customer/month) if that same customer had a value 4 months later. (You probably would have to mess with the table calc settings to get the right addressing of dimensions so that it looks 4 marks away from the current mark within the same customer ID (rather than looking 4 customers away in the same month.) This will be specific to your data and sheet setup.)
You would have 4 of these calcs, each looking a respective 1, 2, 3 and 4 months away.
Then you would have another calc that would sum up (WINDOW_SUM() ) the 4 LOOKUP values, this time summing within the given month, rather than within the given customer.
If you need help setting this up, you'll have to upload a sample workbook so I can show you.