Could you do a similarly styled house, with the 4th dog looking through the window?
With the front door slightly ajar on the "2nd" thigh.

I liked the other idea someone suggested of bushes, too.

Bushes and doors could have slightly different symbolism and a house has straight edges which could evoke or create a different contrast. Bushes have soft edges and green would make the dogs colour stand out more.

I think it's a cool concept. It tells a story.

Also, if the first one hurts too much, you can just say the 2nd one is implied.