If you're fired, the economy's fine, so you can probably find something else in a few weeks. If you have only three or four years of experience, it's highly unlikely that most thoughtful, competent corporate PR honchos would give you a second look, let alone any assignments. So it's generally unwise to spread your wings with so little experience to keep yourself competitively afloat. If your work for someone else clicks, if it's getting real results, and if you're getting frequent calls from quality headhunters and satisfied clients, that's a hint that you know what you're doing and might make it on your own. All of this helps build a track for you to go on.
Business plans are about as superfluous as a flight plan is to a pilot.
You need to know where you're going, you need to know obstacles in your path, you need to plan how to get there while avoiding the obstacles, you need to be sure your aircraft is durable enough to make it through stormy weather, and you certainly need enough fuel on board before you start your engines. He makes progress only when his neck is out. Above all, you must have the courage to go after business, and the wisdom and patience to realize that success is likely to take far longer to achieve than you expect. If you've been laid off or terminated, you'll probably receive unemployment compensation, which can help you start your own business if you continue to meet the job-search qualifications your state sets as a prerequisite for benefits. The more money you have as a startup cushion, the better.