VBA code for Disable/Enable cells

Start by unprotecting the range B2:B5. Then protect the sheet using a password of your choice. Next copy and paste this macro into the worksheet code module. Do the following: right click the tab name for your sheet and click 'View Code'. Paste the macro into the empty code window that opens up. Change the password (in red) to match your actual password. Close the code window to return to your sheet. Make a selection in B2.

Private Sub Worksheet_Change(ByVal Target As Range)
    If Intersect(Target, Range("B2")) Is Nothing Then Exit Sub
    ActiveSheet.Unprotect Password:="MyPassword"
    Select Case Target.Value
        Case "Yearly"
            Range("B3").Locked = True
            Range("B4").Locked = False
        Case "Monthly"
            Range("B3").Locked = False
            Range("B4").Locked = True
        Case "Weekly"
            Range("B3:B4").Locked = True
    End Select
    With ActiveSheet
        .Protect Password:="MyPassword"
        .EnableSelection = xlUnlockedCells
    End With
End Sub