You mean the one listed as a Gonodactactylaceus spp? That's a whole genus and you are not guaranteed to get a fish of any certain size. If they absolutely knew the mantis they were selling you were a max of 4", they would include the species name. They say spp so they can send you whatever they have in stock. You could wind up with a 1" or a almost 5". If anyone asks in the future what kind of mantis you're talking in this case is Gonodactactylaceus spp. The common name "Lime Green" referes to a few different genera, even spearers. There is no fish big enough to outlive a mantis that would be suitable for a 10g. In a 10g, the stakes go up, as it would be harder for the fish to get away.