If they're male it's gotta be Masaccio. Sure he's technically a quattrocento figure, but his contributions to the development of linear perspective should earn him a spot with the big 4, maybe you work that into the story somehow and say he's older and acts as a role model to the rest of the gang. The Trinity in Santa Maria Novella is so damn good. At first I was thinking more along the lines of Ghiberti, but he isn't known for his first name Lorenzo and that name is a little too close to Leonardo. Hieronymus or a more referential Ike (for Jan van Eyck) could be good choices too if you wanted to get a representative of the Northern Renaissance in the gang.
For a female turtle, I'd echo the call for Sofonisba, or Sofie for short if necessary. Artemisia feels like the more obvious choice but doesn't sit right in my mind for some reason. I always liked her self portrait while painting, always a great way of making two paintings in one.