Considering the amount of money invested, the one who invested into Bitcoin is probably Bill Gates because of the following reasons:
1. Bill Gates has not formally announced his investment into Bitcoin but he expressed a positive opinion about it in the past. See reference below for more information.
2. Bill Gates have a total net worth of 86 billion dollars in 2017 doubling that of bitcoin's total marketcap of 41 billion dollars by the time the reference below was written.
3. Considering the fact from reason number two, it is an amount he will not regret to lose and he is the only one who can afford to risk that amount without worrying for his future financial conditions.
4. He has a lot of connections that can help him buy bitcoins worrth 400 million $ easily.
Considering the reasons above it is very likely that it is Bill Gates who bought 400 million $ worth of bitcoin for investment. I think he intends to hold it long term. I might be wrong because there is a chance that he have bought bitcoin way back it's early stages.
References:
https://www.cnbc.com/amp/2017/12/19/bill-gates-in-2014-bitcoin-is-better-than-currency.html
https://www.zerohedge.com/news/2017-06-22/bitcoin-perspective-bill-gates-worth-more-gold-200-times-more