"If the rpm at which the max torque occurs is 4500 rpm, is it safe to assume that the rpm at which max power occurs is twice i.e 9000 rpm?"
No, it is not safe to assume that at all.
"is it usual for automotive engineers to set the upper and lower rev limits at which gear-down and gear-up occur to 4500rpm and 9000 rpm(i.e max torque and max power rpms) respectively?"
No, it is not usual. There are a lot of factors that weigh in when determining shift points. Are you trying to achieve optimum fuel mileage? Optimum acceleration? Smooth acceleration?