Slightly confusing. First you want to calculate time between B9 and Z9, D9 and Z9. And later you say you want to calculate between B9 and D4 until Z9 is entered.
I guess the Z9's in the first sentence are a typo and you meant to say D4.
Assuming you want to use column AA and AB for the day differences, check the following code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Columns("Z")) Is Nothing Then Exit Sub
If Target.Cells.Count > 1 Then Exit Sub
Range("AA" & Target.Row).Value = Range("D4").Value - Range("B" & Target.Row)
Range("AB" & Target.Row).Value = Range("D4").Value - Range("D" & Target.Row)
To implement the code, right-click the sheet's tab and select View code. Paste the code in the big white field of the newly opened window. This window can be closed immediately after.