Yes, it varies on your body size, but drinking 3-4 liters in a few hours has been associated with cases of water intoxication and death. Just over 1 liter in less than an hour can begin to put stress on the kidneys, which may not be able to expel water fast enough. When a person drinks a large amount of water very quickly, it dilutes the sodium levels in their blood, which can cause cells to swell. This swelling is especially dangerous in the brain, where it can lead to severe symptoms like nausea, muscle cramps, confusion, seizures, coma, and, in extreme cases, death.
It can be painful because the water causes the brain to swell, which increases pressure inside the skull. Before a deathly amount, it can bring intense headaches, nausea, and vomiting. It can also be slow, prolonging the discomfort and suffering. Think it through and take care, avoid any excess harm on yourself. I don't wish for anyone to die, especially in a painful way, even if I'm answering this question.