I think Bitcoin even at current levels is an excellent buy. Even at these levels I think it is still cheap and a good investment. Below are few reasons.
Total Bitcoin supply currently is just 16 million and there will ever be just 21 million coins in total. So supply is extremely low.
Reasoning about the demand, at this point, very very few people all over the world know about Bitcoin, and demand because of these very very very few people have caused Bitcoin's price to be over $4000. Imagine what would happen if even 10% of total world population comes to know about and invests in this. Let alone world, even if 10-20% of US invests in this, price will skyrocket.
Even at $4000 levels, Bitcoin's market cap is just $66 billion dollars in comparison with total US stock market cap which is at $22 trillion dollars this is puny. Be sure that someday cryptocurrencies will hit market cap of atleast $1 trillion dollars and majority of which will be contributed by Bitcoin and Ethereum. Those level will result in > $250,000 per Bitcoin.
In summary, I believe Bitcoin is extremely cheap at these levels. If you are afraid to invest, just buy couple of them and sit on it for few years.
Happy investing!