But the path of the righteous is like the light of dawn, which shines brighter and brighter until full day. The way of the wicked is like deep darkness; they do not know over what they stumble.
I've heard sermons preached on Prov 4:18 before. The general gist has been that if a man is righteous, his prosperity will increase and not diminish.
This verse was spoken over me once, in such a way that it seemed to mean exactly what it said, and had nothing at all to do with my finances. So the application was a little obscure. I was reminded of this verse the other day and decided to read it with the verses surrounding it.
I think verse 19 helps understand verse 18. Both verses refer to a path. Both righteous and wicked have a path laid before them. But the wicked's path is shrouded in darkness, which is its distinction from the lighted path of the righteous.
The wicked do not know over what they stumble because they cannot see it. That does not mean that the righteous will not have stumbling blocks in his path--simply that the way will be well lit for him, and he will be able to identify what makes him stumble, and either avoid or conquer it.
I think the point of these verses is not that only the wicked will have problems in their life. But our God will not leave us to wander in darkness.