I’m very sorry, but it seems like there needs to be a bit more understanding about chemical substances. As2H4Se5 doesn’t seem to be a widely recognized chemical substance. You may have altered slightly the chemical formula of another substance or perhaps got confused in the notation of the chemical substance.
Each chemical substance has its own formula representing the number of atoms of each chemical element in a molecular unit of that substance. For example, the chemical formula of water is H2O, meaning that each water molecule contains 2 hydrogen atoms and 1 oxygen atom.
The chemical elements in the formula you provided are As (Arsenic), H (Hydrogen) and Se (Selenium), but the ratio among them in the formula you provided doesn’t match any known chemical substances.
I advise you to review the chemical formula you’re trying to understand. If you provide me with the accurate chemical formula, I’d be more than happy to help you learn more about it.