This the 2nd time I have purchased these (we outgrew the first set).  These are so perfect for a 4-yr-old who loves to wear dresses, but has no modesty yet and loves to play & tumble!  They are short enough to wear under shorter style dresses without showing.  My daughter loves them and wants to wear them all the time.  I think they are too short to wear by themselves (for my personal taste).  So I have to call them her "under dress shorts".  The tiny rhinestone are a nice touch to make them a little more 'special'.  I don't love the price, but they are great quality and they last, so I'd say they are worth it.  I've tried the girls 'bike shorts' from Carters for her to wear under dresses, but they tend to ride up.  These are much more comfortable.
as far as sizing, my daughter generally wears a 5t.  We bought the size 5, and I thought the looked a tiny bit small.  She swears they are comfortable, and they aren't tight or anything.  So order the exact size or possibly size up.