[I]"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?"[/I]
No, it is not safe to assume that at all.
[I]"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?"[/I]
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?