Xiao Zhang and Xiao Zhao do the same job, and Xiao Zhang's efficiency is 1.5 times that of Xiao Zhao.

Xiao Zhao's efficiency is set as 2 and Xiao Zhang's efficiency is set as 3. According to the meaning of the question, Xiao Zhao works 1 hour, and the workload is 2. At this time, Xiao Zhang's workload is 9 times that of Xiao Zhao, so Xiao Zhang's workload is 18. Assuming that after n hours, Xiao Zhang's workload is four times that of Xiao Zhao, there is (18+3n)÷(2+2n)=4, and the solution is n = 2. So the correct answer is C.