Nothing has gone wrong with this car.
This is a very nice luxury SUV. The interior is well constructed and seems quite solid. The leather seats that come standard on this model are extremely nice and well made. The stereo system sounds very good. The gauges and controls / cupholders, etc. are well thought out.
The exterior of the vehicle seems very solid as well. I worried a bit at first because there is a lot of plastic body cladding on the car, but I have had no problems with any of it falling off and it seems well connected.
The engine and drivetrain are silky smooth and seem well mated for most tasks. I have noticed that the motor in this vehicle has somewhat less tork than the Jeep Cherokee that we used to own. Usually this is noticable only when towing something heavy at high speeds, like a boat. Is it a problem? No, not really. With the overdrive off it is hardly noticeable. I have noticed that the gas mileage IS pretty horrible. I think that I am currently getting around 15 MPG in town, 18 on the highway. This can be painful with the price of gas around $2 a gallon...
Despite the poor gas mileage, I highly recommend the purchase of a QX4. It is a fast, good looking, reliable, comfortable vehicle.
I also have a 03 QX4 and the key to improving gas mileage is to drive it gently and look ahead for traffic, lights, etc. A steady 65 mph on the highway, level road, dry weather gives up to 23 mpg. Try a injector cleaner for better mileage as well. Good luck.
It is very sad that Infiniti stopped making the QX4 to start making the QX56. The replacement vehicle is nothing like the original, and has had poor reliability. I love my QX4 and would like to get another one, but the newest one you can get is an 03. I don't mind used, but that model will already be 4 years old in January...Sad...
I totally agree with the above comment that Infiniti should have continued with a QX version of the Pathfinder.
The QX56 is much too large for our needs and the styling is not to our taste.
Using the current Pathfinder as the basis for another QX# Infiniti model would be a win/win for all.
They would have to pull off an amazing styling job because the current pathfinder is incredibly ugly.