Since this is a "4% tax" it sounds like they must be planning to levy it as a sales tax to be collected by any vendors who have to collect New York sales tax for transactions within the state; otherwise it doesn't make any sense to talk about "4%". One alternative would perhaps be that it could be a bandwidth tax to be collected by all of the New York ISP's - which would be more collectible: in most cases, your ISP certainly knows where you live even if (as in the case of wireless) it's only where you receive your bill.
But if it's going to be a standard sales tax, that raises all sorts of other problems. Most obviously, it provides a significant disincentive for companies selling downloads to locate in New York; it would be hard for them to collect tax from some company based in Canada, for example. But it also raises the question of how a company knows who they're dealing with; with many payment options, the customer's location need not be given, and since this is an Internet download if the company does ask for an address it would be easy enough for the customer to enter an out-of-state address to avoid paying the tax, and the company would never be the wiser. If the state requires them to use IP addresses to determine the customer's tax liability, it can often be difficult to determine the exact state for an IP address in a border area or in many other situations, and doesn't even address the problem of proxy servers that might be used deliberately or otherwise to avoid paying the tax.
Since the TFA is rather short on specifics, it's hard to tell how unworkable this might be, though whenever the Legislature - any Legislature - is in session, hare-brained schemes abound. It does sound like they're trying to see just how many people they can annoy with this kind of law.