Bitcoin has a market cap of 70 billion. This is very low when compared with the stock market or the gold market. There are many groups that can buy the total number of Bitcoin in circulation with a price of $4000. They can easily manipulate the market both sides up or down. Bitcoin 70% supply is stored in a very less number of wallets say 5%. Don't you think Bitcoin can easily manipulate and below $1000 is still possible?
It will be quite difficult for them to do so because if they do buy the whole coin in the market, fine, the market price will pump high but who will they sell the coin to at that high price, because it must have cost them fortune too to do so, it will end up becoming useless to them considering the fact that it is a system that is still struggling to gain relevance, so this will also make it impossible for them to manipulate.
Moreover, talking of buying the whole BTC in circulation is very impossible because not everyone bought BTC at bear market, most people bought BTC when it was even $10,000 and majority of them will be unwilling to sell till bitcoin increases above that, so they can only buy the remaining supply which will push up the price up a bit and many believer will keep holding till it reaches $20,000 predicted for next year.