The product information is clear on this point: if the collar causes pressure, numbness, skin irritation, dizziness, or if the pain becomes stronger, stop using it and seek professional advice. These signs should not be ignored, and they should not be expected to disappear if you simply wear the collar longer.
A soft cervical collar should feel supportive, not painful. If you notice one of these symptoms, the first step is to remove the collar and allow the skin and neck to settle. If the symptom goes away quickly after removal, it may be a sign that the fit needs to be adjusted. Check whether the collar is too high, too tight, or placed incorrectly; a collar that is too loose can rub, while one that is too tight can press on nerves or blood vessels.
If the symptoms were strong, happened suddenly, or did not go away after removing the collar, contact a doctor or pharmacist before putting the collar on again. The same applies if you feel dizziness or increasing pain while wearing it. Continuing to use a collar that causes these reactions can make the situation worse.
Also remember that correct size and fit are important for safe use. If you think size 4 is not right for you, ask a professional to recommend a different size rather than trying to adjust the collar yourself. Do not modify the collar or add extra padding unless a professional has instructed you to.