The KDX200 would be a good step up from the DRZ250 as far as woods riding/hare scrambles goes, but wouldn't be ideal for any kind of free style use. I'd probably go with a 250 2-stroke. I'm kinda partial to the green ones but they're all pretty good.
The 4-strokes rock, but there are definite trade offs. For one, they're a lot more expensive to repair when it's time to rebuild them. Secondly, the maintenance is a little more intense. You don't just "adjust" the valves on modern 4-strokes with a cup and shim setup. Then there's the weight factor and kick start vs. electric.
I tried racing hare scrambles on a 4-stroke 400 with kick start. When you flare it out, and you will from time to time, it can be a real pain to get fired up again.
At 5'5" you may want to consider having a 250 lowered an inch or so. Hope this helps