the oldest rocks on earth date from about 4.2 billion years ago. what does this suggest about the interval between 4.6 billion years ago, when the earth started to form, and 4.2 billion years ago?

The Hadean eon represents the time interval between 4.6 billion to 4 billion years back. It is marked by the initial formation of the earth, right from the time of gathering and accumulation of interstellar dust particles and gases, and there was frequent collision between the planetesimals of relatively larger in size. The core and the crust formed and started to become stable and also gradually led to the formation and expansion of the ocean and the earth's atmosphere.

This is how the earth gradually started to form and develop and took the shape during the time interval between 4.6 billion to 4.2 billion years old, and where the oldest rocks found on earth is dated from about 4.2 billion years in the past.