can you already bend on other holes?
If so you should relax and start to experiment using your tongue/throat muscles. You need to control your airflow to bend the reed the way you wan’t it. Sometimes it helps to just drop your jaw while playing the 4 hole. Or you could try a lower tuned harp (e.g. a major) – here the reed are longer and it is easier to succeed bending the 4.
But remember: Relaxation of mind and body is key to get things flowing.