Showing different techniques
Some techniques can more easily be expanded when requirements change, or bugs need to be squashed
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rLogCell As Range
Dim wsLog As Worksheet
Select Case Target.Cells(1, 1).Address
Case "$B$4"
Set wsLog = Worksheets("Log")
Case "$B$6"
Set wsLog = Worksheets("Log2")
Case Else
Exit Sub
End Select
If Target.Cells(1, 1).Value <= 0# Then Exit Sub
With wsLog
Set rLogCell = .Cells(.Rows.Count, 1).End(xlUp)
If Len(rLogCell.Value) > 0 Then Set rLogCell = rLogCell.Offset(1, 0)
End With
rLogCell.Value = Now
End Sub