Let's define the following events:
A: Your first tire is defective.
B: Your second tire is defective.
C: Your third tire is defective.
D: Your fourth tire is defective.
Then, the probability that none of your tires are defective is:
P(none defective) = P(A' and B' and C' and D')
Since the tires are independent events, we can multiply the probabilities of each event not being defective:
P(none defective) = (0.9)^4 = 0.6561
Therefore, the probability that at least one of your tires is defective is:
P(at least one defective) = 1 - P(none defective) = 1 - 0.6561 = 0.3439
So, there is a 34.39% chance that at least one of your new tires is defective.