The US government owning 4M BTC ~
That's easier said than done. To purchase 20% of the total bitcoin supply at the current price they'd need $270 billion and sellers who would give up their coins. And all of that is if the price stays the same and not shoot up making the whole purchase cost $270 trillion. 
Well, $270B is not out of the question when you are talking about the US government, but the goal here is to buy enough to manipulate the market (which is what everybody here seems to want). So they probably wouldn't need to buy 20% of Bitcoin, probably only 5% would do it.
And on the flip side, if Trump got mad at the Bitcoin whales (say they didn't donate enough) he could threaten to outlaw Bitcoin in the USA and send the price down by about 80% in a single day, with just a few words from the President of the US.
"He who can destroy a thing, can control a thing." -- Frank Herbert / Dune
Bitcoin getting into bed with politicians will be it's downfall. Just watch.