You are experiencing the phenomena that scientists call "normal operation." The lower radiator hose is where coolant leaves the radiator after its heat has been, umm, radiated away. If you're driving through winter air it's going to stay really cold.
^^This! It doesn't have to be as cool out as you think to find a cold radiator hose during cool weather. The Qx4 has a tow capacity of 5000 lbs, the cooling system is designed to keep the engine cool when its 110 degrees out while towing 5000 lbs... Therefore this engine has plenty of excess cooling capacity when its cool out, not towing, and when you are using some engine heat for the interior.
The fact of the matter is your engine is staying cool and stable, your heat is hot, its working as designed. It's not unusual to trap a little air while opening up the system, keep the overflow topped up and you will be fine at this point.