How do I (further) lower the pH of a pyrrolidine solution?
A pH of ~ 4 would correspond to a solution of pyrrolidinium chloride.  Use less pyrrolidine or more hydrochloric acid to arrive at that point.

The commercial concentrated hydrochloric acid is 37 % (w/w), this would be a concentration of ~ 12 M, and 164 ml of that is slightly less than 2 mol HCl.  It's very strange that you are seeing a basic pH.