Well we got married on April 18th, 2010 and renewed our vows at CSA on April 22, 2010. lol
Everywhere we went people were suggesting we come back there in 5 years to renew our vows so I assume 5 years is what's trendy now. We tend to gravitate towards multiples of 5 so I think it's just a nice comfortable number for people. Do it whenever you want! It's your life. Nobody made me feel silly for renewing after a few days so I doubt they would think 4 years is too short!