[edit]I'm mostly wrong[/edit]
Your 1/2/3 options are all wrong.
4) Steam
adds VAT onto the retail price, depending on where the customer lives.
Steam is your publisher and your retailer.
Typically, retailers handle VAT. If the game has a price of $10, someone in a country with no VAT will be charged $10, someone in a country with 15% VAT will be charged $11.50. The retailer, who is actually selling the product to the customer, takes care of adding that extra bit of tax to the price, and giving it to the customer's government.
Retailers typically sell products at retail price (plus tax), and buy them at wholesale price from your publisher. In the case of Steam, because they're the retailer and the publisher, there is thankfully no difference between the retail/wholesale price.
If you have a royalty agreement with your publisher (typical these days is 70% for you, 30% for them), then this would probably be splitting the
wholesale sales price. A brick and mortar retailer might buy a new game for $40 wholesale and then sell it for $60 retail -- you'd be splitting the $40 with your publisher. Be aware of these details when reading contracts -- e.g. the Unreal 4 royalties requires you to pay Epic 5% of the
retail price of your game!!
So with a $10 game on Steam, a 15% VAT customer pays $11.50, their govt gets $1.50, you get $7 and Steam gets $3 from each sale.
However, there's also other forms of tax that you have to pay besides VAT. Let's just say for example that businesses have to pay a 30% tax on all income...
Some publishers would give you $7, and then you have to pay $2.10 to the government in income tax, only keeping $4.90 for yourself.
Other publishers will pay the $2.10 to the government on your behalf, and you can keep the $4.90 without worrying.
When you sign a publishing agreement with steam, they'll tell you all the paperwork that you need to fill out, including tax declarations. AFAIK, Steam can handle paying the tax on your income for you, while I think Google Play doesn't. That's just rumors though -- read the information you get when signing on with them.
It also depends which country you're in... Steam is a US company, so they have to report all their income to the USA's IRS. Depending on which country you live in, you could end up being double-taxed, where you have to pay a percentage to the IRS, and to your own government's tax agency again after Steam pays you
To avoid this, you need to live in a country which has a "tax treaty" with the US -- which allows Valve to report your income to the IRS, but then pay the taxes only to your own government (and not actually pay anything to the IRS).
As others have said, an accountant can help with all that...
Back to the off-topic UE4 example -- let's say that VAT is added onto the price so it doesn't affect you, your game retails for $10, you get 70% of that, and you pay %30 in income tax. That means you end up getting $4.90 per sale, but Epic games want you to pay them 5% of the retail price, which comes to $0.50... which means you're actually paying 10.2%
royalties on your income, not the 5% that a lot of people are expecting
So always read the contracts