If someone overworks themselves to the point of death (e.g. sleeping only three or four hours a day while knowing that this is harmful) is this considered to be the same as suicide according to Islam?
Well, if you deliberately work for too long and your preparing yourself to die by doing so, then Yes it is a suicide.
But, even though you're not deliberate intending to die by doing so, but still know that it is harmful for your body, then it is Haram(not allowed in Islam) to harm your body that way.
May the creator guide us all.