He wrote "HI." He's really into spelling words, rather than just writing letters for the sake of writing letters.
The letter writing activity quickly turned sensory. That's perfect for a 4-year-old-though.
We also made some classic macaroni letters. I remember making these when I was a little person.
Sometimes, in my quest to think of amazing, original, creative homeschooling activities that I forget the tried-and-true-oldie-but-goodie activities of my own childhood.