Whats the cause of muscle twitching very often during the day for the last 4 days in my left upper arm?

It depends: Involuntary muscle twitching can be caused by electrolyte imbalances (low potassium, for example); overuse (fatigue), some neurological conditions (writer's cramp among other things) dehydration, and even some medications or discontinuation of medication. If it continues and is a concern see your pcp for a more thorough history and examination.