Restrict Cell Value Macro

I have a range B1 to B4 for user to input value. Worksheet Change Event

However, the range B1 to B4 should not accept any value if the cell on the left hand side is empty.

Eg,
If A1 is empty, B1 should not accept any value.
If A2 is empty, B2 should not accept any value.
If A3 is empty, B3 should not accept any value.

Below code checks for condition in A1 only.

Private Sub Worksheet_Change(ByVal Target As Range)

If Not Intersect(Target, Range("B1:B4")) Is Nothing Then

If IsEmpty(Range("A1")) Then
MsgBox "Please input something under column A."
With Application
.EnableEvents = False
.Undo
.EnableEvents = True
End With
Target.Offset(0, -1).Select
Exit Sub
End If

End If

End Sub

Eg, If I enter something in A1, B2 would accept a value which is not what I hope to achieve. It should be checking for cell in A2 whther it is empty.

Would be grateful for some advice on how to modify the code.

I attached a sample file for reference