Got any pics? What strain is it? Or is it just bagseed?

Most likely it's a hybrid. Even Sativa-dominant hybrids will have fatter leaves at first, and then thin out as they grow. 4 bladed leaves is nothing to worry about either.

14-16in. tall in a month, from seed, is pretty good, but you say it's not bushy. What kind of light are you using and how close do you keep it?