How do I know she's growing up?

Peach is four now.
Having had a recent birthday, there are nevertheless very few stark benchmarks that come with her new age.  As expected, most of her development has been gradual and therefore difficult to pinpoint.  Which is why I'm amazed to notice these pinpointable changes.

She rides a bike.
That is, she rode a brand new, lovingly team-Stein-assembled Disney Princess birthday bike (with training wheels! and a seat for a doll/stuffed animal! and a handlebar pouch for snacks!) several times on successive days, until she almost fell off one too many times while navigating the uneven turn from our driveway onto the front walk.  Now she wants to ride her tricycle again.  We might try again tomorrow, fully strapped with the knee and elbow pads that came with her 3rd birthday Disney Princess rollerskates.  That first day, when she jumped right on and took to it like a fish to water, Daddy and I co-marveled at how tiny she looked perched atop her shiny new ride.  A one-block trek was punctuated with multiple stops for snacks (from the princess pouch, of course), inspecting ants, and picking dandelions.

Her ponies are no longer just the factory-labeled characters.
They are free to be Mommy or Daddy, requesting help from the other to "get the water out of the trunk" [of the car], much like the real-life counterparts.  A "little sister" character is now common in her cast, too, usually being denied the use of a tiara or other personal item belonging to either Mommy or the big sister.

What really knocked me out, though, is the skill she flaunted today.
She can craft familiar animals from playdough.  She's been squeezing and rolling it proficiently for over a year, including some exploration with various plastic apparatuses and molds to make abstract and food-like shapes, but the independent combination of basic shapes to form animals (and even a fairly recognizable pink/purple Tinker Bell) blows me away.  True, she's basing her creations on the included illustrated instructions (from a Crayola Model Magic Pop-a-Dot kit with kangaroo/platypus options), but her ability to translate those flat images into a three-dimensional creation seems like a disproportionately advanced skill.  Maybe it's just Mommy pride.

Like the pride I feel every time she requests and eats a green salad.
I still haven't completely mastered that skill.