P.S....

I realize this thread is years old, but people reference this stuff and when newbies see advice like this at that at the end of a thread they run with it!

Rootbinding is a problem you may see occasionally in a plant that has spent a long time in a small container and there is litterally no more room for root growth. Roots become entangle and grow in circles. It doesn't happen to 4 week old plants unless you planted it in a shot glass!