What is the interpretation of seeing a light in the room in the same spot during the night vigil prayer (Qiyam al-Layl), and this has recurred four times?

There is no definitive explanation for what you saw, but it might be similar to what happened to Usaid ibn Hudair when he recited (the Quran): his horse became agitated, and he saw something like a canopy with lights resembling lamps ascending into the sky. The Prophet, peace and blessings be upon him, informed him that those were angels listening to him. Therefore, you should fear Allah, persevere in night prayers, and not pay attention to such matters.