I'll disagree with most people. I think the whole "buy a shitbox and drive it until it dies" thing is a bit of misplaced economy, depending on your need for reliable transportation.
I am of the mind that you should buy a new/nearly new car and drive that until it dies. Personally, I rely on my car, and more importantly, my wife's car (my wife usually transports our daughter), to get us to work every day. We both had fairly long commutes, which have shortened recently, but any disruption due to unscheduled work on the car is a monster PITA, and further, I don't want my wife and kid stuck on the side of the road somewhere. There's also a large difference between a car you buy new or nearly new and drive for 100k miles, and a car you go out and buy with 100k miles on it. One is a known quantity (assuming you've maintained it correctly and not crashed it severely) and the other is a complete crapshoot.
My advice to you is that you have a reasonable, practical, reliable car, just keep it for another 10 years and you'll be fine. Ignore the fact that you're $4k upside down, because that will fix itself on its own with time. Just take care of the car and don't worry about it.
The only exception to this advice IMO is if you do not need reliable transportation (bike/public transportation to work or work from home or retired, for instance). If you can tolerate unexpected downtime, by all means, you can cheap out on a car, but for most people that isn't a realistic option.