Some weeks ago I started a new team with 4 shamans. Leveled them to about 20. Then I realized that since I have the hardware to run one more it was just silly not to level one more so I started over from lvl 1 again (I wanted to change names and races on all too).

Now I'm soon lvl 50 with my new 5-box team and I do not regret this desicion yet. I also think that leveling 5 chars goes faster than levling 4 since you both can do instances faster,and often I find that I can oneshot mobs with my 5-box where my 4-box would have had to nuke one more time(causes more downtime)

If I want to switch to 4-box later I could allways just retire one of my accounts, but if I had started out with just 4 I would have to level all the way from lvl 1 once more :P