how far is it from where room one is to room 3?
Idea set up is to have the 4500 central to the house or building in relation to each end of the house and on the main floor. Since you have the 4500 at one end and need singal at the other, the DAP should fill this other end in nicely. I would prefer to run the 4500 as the main host AP and then run the DAP as a 2ndary AP since you can run the same SSID name and password. The only thing you need to ensure is that the channels is different on the DAP from the 4500, so if you run channel 11 on the 4500, run channel 1 or 6 on the DAP. I have my main host AP and 2 other APs at my place.
I prefer to use Mixed G and N modes cutting off B mode if you don't have any B mode devices that need that mode, theres no reason to broadcast it if your not using it.