Great job at the savings at your age. I agree with the suggestion that you should be capturing the extra 15% gain through tax avoidance by putting everything into a traditional 401k and nothing into a Roth 401k. Once that's maxed, then contribute as much as you can, up to the $5,500 max to a Roth IRA.
Overpaying for housing and cars are two things that will for sure extend the time it takes for you to get to FI. MMM preaches this endlessly. $15K is WAY too much to pay for a car on your salary. I know, shocking statement that is counter to conventional thinking. But, you are in a forum titled "Ask a Mustachian" and conventional thinking causes most Americans to be broke on a median salary (close to yours).
I agree, $4K may be too low for the conditions of cars in your area, but you should be able to shop around carefully and pick up a fine, clean, safe, and reliable vehicle for no more than $8K (see article below, and shop around). I personally would focus on highly reliable brands (i.e. Japanese and Korean, rather than European or American). Save up for it and pay cash. Don't stop or slow the automatic pilot savings that you have established. That will mitigate the amount you spend. Don't even think about pulling money from your Roth to pay for it. The $8K 'asset' will be worth less next year, while the $7K that you 'saved' from averting the overspending may be worth 6-12% more in a S&P500 index fund. Think about that $7K savings making you $50/month ($7,000 * 8.6% = $600 / 12 = $50/month). Get all of those green monetary soldiers working for you!
http://www.mrmoneymustache.com/2016/01/28/the-man-who-gets-his-cars-for-free/