Are you saying the sub doesn't have it's own amplifier?
The answer depends on your receiver, but most aren't set up to output the main LCR signal to more than one channel.
In any case, you cannot connect two channels to one speaker without risking extreme damage to the amp--there's nothing to stop the output from one channel from just going on into the other channel.
Is it still in a car shaped enclosure? It would be interesting to hear this in a room environment. Very different designs.
Basically, the safe thing to do would be to purchase a separate amp for it. Then you can hook it up 'properly' to you receiver and have control the low frequencies it receives.
If you try to hook it to even just a single channel (say assigned as a front) on you Denon, it's going to receive the full frequency range instead of just the low frequencies that it would be limited to if it were attached to the proper sub output.
That means, whomever built your sub better have included a built in crossover component to block those higher frequencies, as the Polk driver by itself, just wasn't built for that and it will sound awful, at best.
So if your going to the trouble of building a sub enclosure with a crossover, might as well add the built in amp as well and do it right.
Note: This isn't meant to discourage you. Building and experimenting is fun.
If you hook it up to the sub-out and use an external amp, the resistance is irrelevant to the sub out on your receiver. The 4 ohms will be a factor between the sub and the amp.
I assume you are mentioning a PC PSU to the amp cause the amp is also an auto variety.
I would suggest that you do a bit of Googling on home made sub building. There is a lot of good info out there on building subs and connecting them properly in a simple manner.
It's not that you can't make all this work in the end with auto parts if that is your goal but, respectfully, I think it's adding a bit of extra complexity for a first project.
Using a 12-gauge speaker cable to your sub won't lower the resistance presented by the subwoofer to the amplifier. The impedance of a speaker is largely determined by the voice-coil windings of the driver in the sub, and the crossover, if you decide to build one or buy a simple analog crossover.
Of course, it's always good to use a 12-gauge speaker cable so you don't raise the resistance (and waste power) presented to the amplifier. For short runs, you could even use thinner cable like 14-gauge or 16-gauge, which would not significantly increase resistance.