What cocktail can I make with gin, pineapple juice, watermelon juice, and peach schnapps?

It sounds like you're looking for a cocktail that combines gin, pineapple juice, watermelon juice, and peach schnapps. While none of the listed cocktails exactly matches all these ingredients, the Fuzzy Melon and the Summer Gin Granita can give you a delightful taste experience that's close to what you have in mind.

The Fuzzy Melon cocktail is a standout choice, featuring a vibrant mix of vodka, peach schnapps, pineapple juice, orange juice, and watermelon liqueur. While it doesn't include gin or watermelon juice specifically, it captures the fruity, refreshing essence you seem to be after with a notable twist from the melon liqueur and the harmonious blend of sweet and tangy flavors. With a rating of 4.35/5, this shaken cocktail would be perfect for a tropical-themed gathering or simply a hot summer day.

Alternatively, the Summer Gin Granita showcases a delightful preparation that includes lemon juice, gin, apple vodka, elderflower cordial, and a generous amount of watermelon juice. Though it lacks pineapple juice and peach schnapps, it still carries that refreshing, summery vibe, and with a ratings of 4.37/5, it’s an excellent option for those who prefer a more complex mix of botanical and fruity flavors. Featured on the BBC's "Something for the Weekend," this cocktail requires some prep work but rewards you with an exceptionally refreshing experience.

Both of these options offer a refreshing and fruity experience that could be close to what you're envisioning. Enjoy experimenting with these vibrant and summery drinks!