As we saw in the comment following verse 2, Mormon is equating righteousness with the concomitant blessing of prosperity. Here he makes that moral completely clear. In verse 4 we see that they begin to prosper in the land. There are various blessings that come from the foundational promise. Peace is one, and along with the peace can come the development of prosperity. Thus in verse 4 we see the benefits of the peace. There “was greate order in the land.” Therefore they also “began again again to propser and to wax great.”
In verse 5 Mormon makes certain that we understand the spiritual connections he is highlighting. They have become great, and they will be great, “except they should fall into transgression.” There is a direct connection between peace, prosperity, and righteousness. Those things go together. It is the rejection of righteousness the reverses the trend and leads to destruction. This is one of the themes that Mormon hammers home over and over again in his narrative.