Although this is common in the first few days after breast surgery, by 4 weeks, most patients should have reabsorbed any "extra" air or fluid in the pocket. But I have had patients notice (and sometimes worry) about a breast "sound" that is almost "felt" as much as it is heard. The French call this "Bourdonnement" and this literally translates as buzzing, vibrating, rubbing, etc. This is actually a friction rub, and physicians learn this physical sign in their first medical school classes in which they listen to chest sounds and literally hear irritated, inflamed lung tissues sliding roughly against the inside of the chest cavity, causing the half-heard, half-felt sounds. In your case it is the slick surface of the implant sliding against the incompletely-healed tissue capsule that causes this "crinkly, crispy" sound.
This is entirely normal at this point in your recovery, but I would consider asking your surgeon for an anti-inflammatory such as Celebrex for a week or two, or perhaps a course of ibuprofen. This may be all you need to reduce or eliminate the sound. Inflammation may also be a possible predictor of early capsular contracture, and a leukotriene inhibitor such as Singulair or Accolate may also be a consideration. Best Wishes!