Last day as a three year-old

I can't believe Koo is turning four tomorrow. Last day he will ever be three and I'm feeling kind of tragic about this. I feel like he grew up so fast and I can't seem to keep up with him. I still see him as a toddler. At this moment, he is building a puzzle and really likes to do everything with gusto. I love that most about him and can't wait to see him as a four year-old.