N.
I had the same problem with my daughter when she was that age. She was very tall but thin. What we did was buy a lot of skorts and shorts and just kept an eye out for leggings and pants that were longer than usual in the 2t and 3t sizes, or "skinnier" size 4 items. A lot of times, her pants looked more like capris or "high-waters", but she wore her clothes very well and they always looked very cute, even if they were short. Cute shoes and socks help too. :-)LOL
You can also sew a cute bottom "cuff" on short pants to make them longer. There is that eyelet lace stuff or a cute coordinating fabric you can ruffle at the bottom of a pair of jeans that will make them 1 to 2 inches longer. If you go into those real expensive little girl boutiques, you can see how they do that all the time and they charge a fortune for a little pair of jeans with a little added fabric at the bottom and maybe a matching bow belt.
Also, and you may do this already, I always washed my daughter's clothes in cold water and never dried her clothes in the dryer. This helped keep any shrinkage to a minimum so she was able to wear her clothes much longer that way.
Good luck!