$3-4K is a lot of money that can also go really quick if you don't have a plan.
A few questions to help guide the process.
Do you plan on doing the work yourself?
Are you willing to learn the details of RMS vs peak power to help build a quality system
Are you comfortable with removing panels off your car?
Are you comfortable with tools in order to build a quality subwoofer box?
If not, do you have a friend that has previously put together systems that would be willing to help?
If not, you will probably need to find a reputable shop. Understandably, your $3-4K won't stretch as far due to the cost of labor.
What are your real goals?
Listen to high quality music?
Impressive friends with a rattling trunk?
Do you want to maintain trunk space?
Reduce road noise to increase your music experience?
Just be loud?
Any reason why you specifically want 2 12's?