If statement

You can do what you're asking by using a macro. Formulas won't work, since when you replace A4:Y270 with 0's the formulas would disappear.

The following code checks, upon opening the workbook, to see if Sheet1 cell G1 is greater than Sheet1 cell G2. If not, nothing happens; if so, A4:Y270 are all changed to 0's.

Private Sub Workbook_Open()
    With Sheets("Sheet1")
        If .Range("G1").Value > .Range("G2").Value Then
            .Range("A4:Y270").Value = 0
        End If
    End With
End Sub