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.
