Assuming this is for choosing which jobs are worth applying for, you want to choose the highest sensible number. To be more specific, assume the highest number you can, limited by whether the interview will expose your inability to do the job. If you're not sure, assume higher. At worst, you'll turn up to the interview, find out you can't do the job, and learn from it. (e.g. "I didn't know this, so I couldn't get the job. Now I can go and learn more, so I can get the next job that asks this")
For example, if straight out of uni, you think "I need to put down 4 years experience to get that job", then when you're asked in an interview, "so, what did you do in your last job", and you describe your hobbywork or uni project that, while valuable from a skills perspective, nobody would pay you for, it immediately gives the interviewer a "mistrust this person" vibe. At which point you're in trouble. Similarly, nothing infuriates an interviewer more than hearing that the "professional" C++ programmer doesn't know what adding #include at the top of a file does.
If you think "I actually can do that job that says at least 4 years experience", then instead of saying "I have 4 years experience", put into your resume the equivalent of "Here's what I've done that is the equivalent of the 4 years experience you asked for."
For your resume, I'd recommend not putting a number down. Instead, give a list of the last 6 years (2007-2013), including personal projects, major uni projects/courses, basically any work at all. This gives me your six years experience, and I can then decide if that meets my requirements. If you're really worried about the number, add a sub-heading above that list that reads "Experience (6 years, 2007-2013):". Honest and the largest possible number.
Edit: As pdr says, it's important to highlight actual employment from pet projects (either via 2 lists or via highlighting the commercial), although different companies would have different policies on where things like short-term employment, off-the-books IT support, and internships lie.
Important: Leave no gaps. This means that if you spent the last two years working in a retail store while playing Halo in the evenings, put the store work in. If I notice a two-year gap post-uni, I can at best assume you did nothing of value. If I learn you spent two years working retail, I now know you can hold down a job.
If you actually did spend 2011-2013 playing videogames and eating pizza, then putting it down as something along the lines of "A journey of 'profound self-discovery' in which I learnt that I actually needed to achieve something with my life and that X-Box Achievements don't count." is still better than the gap, which reads as "I don't want you to know I did nothing during this period where I can't describe what you did. Now give me money."