With every flip of a coin, the chances of flipping heads or tails are the same; 50% chance of either.
For your question, you are asking what the probability of getting heads, tails, heads, and tails (in that order) is. To find the probability, your formula will be 1/2 multiplied four times. 1/2 because of the 1 side that you are aiming to flip (heads on the first flip) divided by the amount of sides there are on a coin (2). Because you are flipping the coin four times, you will multiply the fraction four times. 1/2x1/2x1/2x1/2 Once this is multiplied, your answer is 1/16.
The probability that a coin will be flipped 4 times and the result will be HTHT is 1/16.