Select CustNumber
From Invoice
Where 
EXISTS
(
  Select CustNumber
  From INVOICE x (NOLOCK)
  Where x.CustNumber = Invoice.CustNumber and x.InvNumber > Invoice.InvNumber and (x.OrderDate - Invoice.OrderDate) <= 90
  Group By CustNumber
  Having Count(*) >= 4 and ((MAX(OrderDate) - Min(OrderDate)) > 30)
)
Group By CustNumber
Order By CustNumber