Unhide / Hide worksheets based on drop down menu value

Total novice to this VBA guys so bear with me.

I want to hide / unhide sheets based on the value the user selects from a drop down menu. There are 3 choices possible in the drop down 1,2,3. The drop down menu is is cell D4
When "1" is selected I want to unhide sheet "Cat 1 7.5 Months" and keep "Cat 2 10 Months" and "Cat 3 12 Months" hidden
When "2" is selected I want to unhide sheet "Cat 2 10 Months" and keep "Cat 1 7.5 Months" & "Cat 3 12 Months" hidden
When "3" is selected I want to unhide sheet "Cat 3 12 Months" and keep "Cat 1 7.5 Months" & Cat 2 10 Months" hidden

Read lots of post and tried to implement and modify code to suit my needs. All failed so far....

HELP!! Please

Guys I sorted it eventually


Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$D$4" Then

If Range("D4").Value = "1" Then
Sh_1.Visible = True
Sh_2.Visible = False
Sh_3.Visible = False
End If

If Range("D4").Value = "2" Then
Sh_2.Visible = True
Sh_1.Visible = False
Sh_3.Visible = False
End If
If Range("D4").Value = "3" Then
Sh_3.Visible = True
Sh_1.Visible = False
Sh_2.Visible = False
End If
End If
End Sub