The result is OK. Visually, I would say the angle between the two vectors is approximatively of 30 degrees.
Remember that the angle between the two vectors is computed from the (0, 0) position. That's not the angle of the line between the two vectors.
To get the latter, I would translate pos1 to (0, 0) and compute the angle between the horizontal axis and pos2.

1. PVector pos1, pos2, pos3, pos4;
2.  
3. void setup() {
4.   size(400, 400);
5.  
6.   pos1 = new PVector(50, 50);
7.   pos2 = new PVector(width-50, 100);
8.   pos3 = new PVector(width-50, 200);
9.   pos4 = new PVector(50, 300);
10.   
11.   noLoop();
12. }
13.  
14. void draw() {
15.  
16.   quad(pos1.x, pos1.y, pos2.x, pos2.y, pos3.x, pos3.y, pos4.x, pos4.y); 
17.   
18.   float a1 = PVector.angleBetween(pos1, pos2);
19.   pushMatrix();
20.   translate(pos1.x - 10, pos1.y - 10);
21.   rotate(a1);
22.   line(0, 0, 100, 0);
23.   popMatrix();
24.  
25.   pushMatrix();
26.   translate(pos1.x - 10, pos1.y - 10);
27.   rotate(radians(30));
28.   stroke(#FF0000);
29.   line(0, 0, 100, 0);
30.   popMatrix();
31.  
32.   PVector pd = PVector.sub(pos2, pos1);
33.   float a2 = PVector.angleBetween(pd, new PVector(1, 0));
34.   pushMatrix();
35.   translate(pos1.x - 10, pos1.y - 10);
36.   rotate(a2);
37.   stroke(#0000FF);
38.   line(0, 0, 100, 0);
39.   popMatrix();
40. }

> To begin i want to draw a line in the direction of pos1, pos2.

the vector from pos1 to pos2 is simply (pos2 - pos1), no need to calculate angles.

PVector.sub(pos2, pos1);