It depends on how much of a jungle you want. Are you hoping to leave a pathway through the middle so you can harvest without stepping on the plants? I usually space cantaloupes and muskemelons 3-4' apart when growing them on the ground. However for about the past 15 years, I've trellised all my cantaloupes, muskmelons and other assorted types of melons (including icebox watermelons) and, for me, 1' spacing for cantaloupes and muskmelons when trellised works just fine. Putting up simple woven wire fencing on 8' tall green t-posts lets me grow a lot more melons trellised than I can grow if I let them roam on the ground.

Thanks, Dawn. I think I am going to plant Sugar Baby watermelons instead. I am thinking about 4 plants in this space.

That should be fine for the Sugar Babies. You might even be able to squeeze in six. Sorry, I can't help it. I'm always trying to stuff 10 lbs. of mud into a 5 lb. bag, so to speak.

So I'm thinking about having 4 hills about 30" apart in a square, each with one sugar baby watermelon plant.

Sounds great! I hope you get a lot of melons.