Nothing ... go ahead. If you fail - Epic is not going to miss your token payment. But if you manage to succeed their lawyers will bleed you dry for much more than the 5% of the revenue. Are you going to take your chances?

20 bucks a month over the duration of a development cycle could be pretty significant. This will basically allow people to develop their game to the point where success or failure could be predicted, and Epic will miss out on the development cycle revenues from the failures.

"You can cancel your subscription at any time and keep using the engine, though without monthly updates."

from: https://www.unrealengine.com/register

Even if you cancel your subscription you are obligated by the original agreement to give Epic a royalty that is %5 of gross revenue.

Is it possible to disguise that the game was built with Unreal Engine 4 or is that something that would always be obvious?

Never touched UE, but a few things - engines do have their distinct feel just from gameplay/graphics/shaders etc.

And even if it is possible (which I doubt) any team that is so skilled will just fork the 20 USD

I believe the EULA of the subscription forbids sharing of the source code with non-licensees. So whoever publicly shared the code has violated the EULA, after presumably having given EPic their credit card and personal info. I think this will be taken down shortly. Someone using this code would have to contend with Epic's lawyers. I hope this doesn't ruin the very liberal UE4 licensing for those who abide by the agreement.