You can use the query as:
SELECT COUNT(*) FROM Users
to get the count.
The primary key of the row should be used to identify a particular row.
If you wanted to return Row_Number() in your main query,
SELECT ROW_NUMBER() OVER (Order by Customer_Id) AS RowNumber, Field1, Field2, Field3
Then when you want to go 4 rows back then you can take the current row number and use the following query to determine the row with currentrow -4
FROM (SELECT ROW_NUMBER() OVER (ORDER BY Customer_id) AS Row, Customer_Id
FROM Users ) users
WHERE Row = CurrentRow - 4