There are 10 coins in a bag. They look the same but four of them are fair coins and the other six are not.

For the six biased coins, if you flip any one of them, the chance of getting a head is 0.7.

Now you pick a coin from the bag, flip it (the same coin) four times and get two heads, and two tails. Based on this observation, do you think the coin you picked is a fair one? Why or why not?

Please include the formulas you have used to reach the conclusion.