is it necessary to replace all 4 tires

Hello, if your SRX is the "All Wheel Drive" equipped model, I'd suggest replacement of all 4 tires at once, as tires with highly dissimilar outside diameters (such as different brand, models or wear patterns) running at the same time can put abnormal and undesirable loads on the drive train components. If it is only a front wheel drive model, you can replace just 2 at a time, but be wary, again, of size, load and speed rating and even brand and tire model -- matching is highly recommended, and for size, load and speed rating is required.