These can be botters, and they can not be botters. They are surely high levels and hunting demons or infernalists. They have always many backpacks of potions. And they are as called "No-Lifers". They are just sitting whole day on tibia.
The exp gathered doesn't necessarely come from 1 day. Gathering that much experience in one day may also be very expensive, if you for example want to get much exp/hour you may as a sorcerer use the 'Hell's Core' spell to kill Frost dragons, which isn't profitable at all but it gives much experience. this may give far more experience than four millions a day.