with open 向文件的某一固定行，追加内容

with open(r"D:RSU地图制作1-南京111111","r") as f:
    lines = f.readlines()
    lines.insert(3,"向第四行，加入的内容") 
    s = "".join(lines)
with open(r"D:RSU地图制作1-南京111111","w") as f:
    f.write(s)