The 4" Cosmic Neutron Nano is a unique mini disc because every one of them is different. They're the same Neutron plastic you love but with that cosmic burst sometimes called a swirl. The little extra price for this mini is because of the extra work done by MVP to make them all so colorful.
A mini is used for marking your lie when you're playing a competitive round of disc golf. They go right in front of the disc and mark where your foot is supposed to be for your next throw. You'll see many of the top pros carrying them in their pockets or in their bags/carts.
A mini is up there on the list of must have accessories for a round.
Note: With cosmic neutron plastic especially, things like knit lines are unavoidable and each disc will end up with small quirks and variations that make each one truly unique.