It's kinda small for > 5 weeks.
And the internode distance is already tight.

So as long as it gets plenty of light, and you limit the container size, it won't outgrow your 5' cabinet.
If you just want to try topping, or fimming, for the experience, I'd wait for another set of leaves.
And while you are waiting patiently, read yer ass off.

Here's a good start:

http://cannabis.com/growing/training-show-me-how-to-do-early-vegetative-pruning-and-fim.html

You will find everything you want to know in the first link of my signature.
Enjoy, and Aloha,
Weezard