Firstly, let me start by saying that the essay score is not added to the verbal section and is optional. Here are some other details:
Based on the documents presented by the college board, my presumption is that 4s in each of the above categories would be the national average. This is an assumption based on the fact that the following image from a college board document shows the mean score for the overall SAT score.
In order to get a 4, you are actually scoring a 2 from two separate scorers. The standard for receiving a 2 (combined 4) for your writing is as follows:
For the average score on the main test, according to the statistics released from the May exam, the average score is 1020 (510 in each section). We shall see what comes out in later months, but that is the average according to the College Board.