If you can’t see anything clearly through your telescope using your 4mm eyepiece, try using your lower-power 20mm eyepiece instead. Even with a lower-power eyepiece like the 20mm, a view can be blurry because of the earth’s atmosphere. Heat waves and high-altitude winds move air around and cause differing temperatures of air to mix. This makes the air act like a weak lens that interferes with the light from a planet or a star by defocusing it. On nights that are like this, the situation gets worse when you magnify it using an even higher power eyepiece like your 4mm.
Always start with the lowest power eyepiece (the one with the highest number in millimeters printed on it). It is much easier to focus and has a wider field of view making it easier to aim the telescope. Look through the eyepiece by placing your eye just behind it, not right against the lens. Now turn one of the two knobs to the side or below the drawtube, first one way then the other until the object is in focus.