4-4 point, double low approach
When White makes an approach move to the 4-4 point with , it is common for Black not to answer in the corner, but play tenuki (or make a pincer on the left side). If White gets to play a second move here, the most usual choice is a second kakari from the other side with (or a). Such a position is called a double approach.
The most common Black response to the double approach is to play at a. See 4-4 point, double low approach, attach.
Black b will most likely lead to a black wall. See 4-4 point, double low approach, outside diagonal.
It is very uncommon for Black to tenuki, because it is considered poor to allow the shutting-in play. (After a White play at b, Black just barely lives in the corner while White gets superb influence.) See 4-4 point, double low approach, tenuki.