4:18 path of the … light of dawn. The path of the believer is one of increasing light, just as a sunrise begins with the faint glow of dawn and proceeds to the splendor of noonday.
MacArthur Study Bible
4:18–19 The path of the righteous is the way of wisdom (v. 11). dawn … full day. The image here is of ever-increasing brightness, from first light until noon. The path of a person refers to the moral orientation of his or her life (v. 14; cf. 2:8, 13, 15, 20; 3:6; Ps. 25:4). It is this that shines brighter and brighter, i.e., keeps increasing in the way in which it displays God’s light (cf. Ps. 19:8; 119:105, 130; Prov. 6:23). This contrasts with the way of the wicked (4:19), which is the way of “evil” (v. 14). The person whose life is oriented toward evil will stumble through life in deep darkness.
ESV Study Bible