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.
No it's not suicide. Suicide is to inflict yourself with harm that leads to immediate death. Sleeping less has nothing to do with self-infliction.
But you should be aware that your body has a right over you and you should take care of it. Resting it properly will help you work more.
A person shouldn't torture on his body; his body also has haq since it's gifted to him by Allah. So everyone should take care of themselves.