Can someone please help me change this crochet pattern?

If you make the 4th leaf of the clover, you may find you need to alter how the stem is attached. I often adapt things on the fly, so I'd suggest using scrap yarn (or good yarn that can be undone and redone without damage, like Red Heart Clic or Lion Brand Vanna's Choice) and trying one. Make your 4-leaf clover, and see how the stem does. If it still works as they instruct, great. If not, you may need to make a separate one and attach it to the center. That looks like a really fun simple pattern, by the way.