To find a specified term in a geometric sequence, you can use the formula: an = a1r^(n-1), where a1 = first term, n = position number of given term, r = common ratio, and an = value of term in given position.
Since we're only given a4 and r, I'm assuming a1 would be 1 since that would correspond to the rate (If we start off with one, a2 would be 1 * 2 = 2, a3 would be 2 * 2 = 4, and a4 would be 4 * 2 = 8). So now just plug in the numbers for the variables and solve.
a13 = 1(2)^(13-1)
a13 = 1(2)^(12)
a13 = 1(4096)
a13 = 4096
The 13th term of this geometric sequence would be 4096. I hope this answers your question.