hard-wire 4-port usb hub to my car's 12v switched cig outlet
Fundamentally, it would be easy to do. 4 Port USB hubs normally require 2 Amps of 5v power. You can get 12v to 5v 2+ Amp supplies quite easily. They can be wired in parallel to a cigarette lighter. Cigarette Lighters are typically on their own 10 to 15 Amp fused circuits, and most are switched, so they are only powered when the car's ignition is set to ACC/On.

BUT from your question and comments, you have a 12v DC to 120v AC inverter, and plug in a home/office usb hub power supply. You COULD replace the cigarette lighter with the inverter, wiring it in. Some cars even come with that as a standard option. Just remember, this is atleast 120W of energy on the car side, and 120v at the inverter side, so wiring could be dangerous. Here is an example of a large 400W inverter wired in (cable to battery) and modded to the center console of a car. Smaller single port inverters could just replace the cigarette lighter without needing to run a cable to the battery, using the cigarette lighter fuse and it's own fuse as the protection.

That said, the three easiest, probably cheaper ways of doing it, would be getting a 2 amp 12v to 5v power supply and wiring that to your usb hub, or getting a 4 port usb car charger, or getting two 2 port usb car chargers for like 5 bucks and wiring those in. People Hardwire usb ports in their dash or center console all the time.

BUT the MOST IMPORTANT THING IS, making sure that the circuit you use, is switched. If it is not, that means the circuit is always powered, even with the car off and key removed. If you can use the cigarette lighter like that (car off, key out), you run the risk of the power supply or inverter you are using to drain your battery. Replacement car batteries arn't cheap, and at best, having to get jumped in the morning would ruin your day.