Well, You're in a tight spot right now.
At 5'4" and being aggressive, you'll be disappointed in an 85, or 150, even if it has bigger wheels.
How much taller will you be next season?
I'd suggest trying out a Honda CRF 250 R, and a Honda CR 125. Right now, they both will be too tall for you to reach the ground well. However a lowering kit will fix that quickly! (google search for "kouba link").
I'm pretty sure you won't run out of power!
Whether to get a 4 or a 2 stroke depends on you. 4-strokes have a reputation for being harder, and more expensive to maintain. If you're not mechanically inclined, then the bills are going to pile up quite quickly.
I personally prefer 2 strokes, as most of the maintainence (never did learn to spell that correctly?) is easy to do even though I'm not completely gifted in the area of mechanics, and very cheap in comparison to a 4-stroke.