There are a total of 4 + 3 + 2 + 1 = 10 marbles in the bag. The probability of picking a red marble is the number of red marbles divided by the total number of marbles:
Probability of picking a red marble = 4/10 = 2/5
Therefore, the probability Aisha picks a red marble is 2/5 or 0.4 when written as a decimal.