In horsepower, the fourth-generation is significantly higher powered than the mini, but in the same way that folks--myself included--have chosen a less powerful MacBook Air over the heavier MacBook Pro, I don't think that's necessarily going to be the major distinction for many.
Finally, what about the third-generation iPad users among us? As an owner of a third-generation iPad, I can't honestly see much of a reason to upgrade. The performance improvements are nice, no question, but I didn't run into any apps that were overtaxing the third-generation model and really required the fourth-generation iPad. If developers start building apps--and especially games--that take advantage of all the power the fourth-generation iPad has to offer, that equation might change. But right now, there's no rush unless you need to have the latest and greatest, or you want to move your household over to Lightning-only.
Given that the fourth-generation model surfaced just six months after the third-generation, it makes sense that this iteration is a more modest improvement on its predecessor.
Yes, the fourth-generation iPad may have been overshadowed by the announcement of its svelte new sibling, but that's no reason to look down your nose at the more zaftig model. It may look more or less identical to the third-generation model, but make no mistake, it's the most powerful iPad yet, and a harbinger of things to come. Simply put, the fourth-generation iPad is a case of Apple putting its best iPad forward.