Chemistry why isn't there an OH4 element since H2SO4?

I understand the Octet rule and everything. What I don't understand is lets take an example of H2SO4. Sulfur creates 6 covalent bond. 2 double bonds with 2 Oxygen and 1 single bonds with 2 oxygen both of which have an another bond with Hydrogen (Hydroxyl group). Sulfur is the element of Group 6 same as Oxygen which means valence shell has 6 electrons. So why does Oxygen have H2O in which Oxygen have a covalent bond with 2 Hydrogen and the remaining electron on "O" are just there, in other words why not H4O. and why isn't it stable it there is such a molecule.