When you are driving around in the city (70km/h and less), there's really no need for you to be in 4th. By leaving the gear selector in 3rd, there is one less gear to downshift when you need to GO, I do it all the time in my wife's automatic car, it makes a big difference if you're already rolling and want to accelerate. If you're driving at a constant speed and not in stop and go traffic, there's really no need to lock it in 3rd. It also will make no difference from a dead stop, only in situations where you are in 4th and need to get into 2nd quicker.

Alternatively, if I'm in 4th and anticipate needing to speed up, I'll slip it in 3rd before I start my pass. It's not going to hurt anything.