I'm not quite sure whether you want to enable the button or set the focus to
it. Hopefully this example should help either way ...
Private Sub Text0_AfterUpdate()
ToggleEnabled
End Sub
Private Sub Text2_AfterUpdate()
ToggleEnabled
End Sub
Private Sub ToggleEnabled()
If Len(Trim$(Me.Text0 & vbNullString)) > 0 _
And Len(Trim$(Me.Text2 & vbNullString)) > 0 Then
Me.Command4.Enabled = True
Me.Command4.SetFocus
Else
Me.Command4.Enabled = False
End If
End Sub