Your maths is out (not that it affects your underlying argument):

If I had a time machine though, (bear with me), the smartest thing might be to go back in time to when Bitcoin was worth $4.20 and buy a million dollars worth. Upon returning to the present, those Bitcoins would be worth $4,200,000,000

A million dollars of bitcoin at $4.20 would get you 238095.238 BTC. Back in the present (assuming a current price of $4200 per BTC), this stash would be worth $1,000,000,000 (not $4,200,000,000).