Depends what you were doing in the 2 hours between the tests really. If you were eating cookies or if you were just having a nice sit down for two hours a change from 8 to 4 might seem unlikely, if you were out cycling for two hours and not eating anything, then it's perfectly possible. Blood sugar can and does change. Two hours isn't so quick for a 4mmol/L change that it immediately suggests a monitor error, to me anyway, without a bit more context on what it is you were doing and why it is you are thinking monitor error.