The wicked are not so, but are like chaff that the wind drives away. (ESV)
The Psalms are beautiful Hebrew poetry. They have different rules of meter and rhyme, but one consistent them in Hebrew poetry is parallelisms and contrast. With parallelisms they will use different illustrations and words to repeat the same theme. We saw this in the first three verses of this first Psalm. After describing the righteous, or "blessed" man, verse 4 makes a sharp transition to contrast.
"The wicked are not so." The contrast of the blessed man is the wicked man. Interestingly, the opening verse declared that the blessed man does not even walk in the same places as the wicked. They are not the same, and they do not associate as if they are. In verse 3 the Psalmist used an illustration from nature to describe the blessed man; a lush, fruitful, strong tree. In verse 4 the Psalmist also uses an illustration from nature. The wicked are described as chaff.
If you have an agricultural background you know what chaff is. Dr. John MacArthur gives us a simple definition: "A frequent OT word picture from harvest time for what is unsubstantial, without value, and worthy only to be discarded" (Dr. John MacArthur, Online Study Bible Notes). Consider the contrast. The blessed man is a strong tree. The winds may blow, but it stands strong in righteousness. The chaff? They are blown away. The unrighteous will not stand in the day of Judgment.
What are you? A tree of righteousness, enjoying the blessed relationship of God? Or is your life being blown away as you chase after wickedness? Only following God is a life of blessing. If you call to him he will plant you by streams of His righteousness.