Between now and achieving that 4%, focus on excelling at the things you find the most interesting and building relationships in your industry so that once you cross the freedom line you can take on the occasional consulting gig to keep things interesting.
(I'm pushing 40 now and sincerely wish I had done the above when starting out in my twenties)
The national debt is only 100 billion dollars, and is there primarily to ensure smooth trading with our trading partners. (It was roughly doubled due to liquidity measures in 2008, but supposedly some smart bond trading ensures that this won't have any dramatic effect with regards to making or losing money).
Famous last words...
Also, some historical data from the S&P 500:
Focus-wise maybe not.