Take a look closely at the colmap distortion model and the nerfstudio distortion model. They are equivalent up to k3. For fisheye cameras, you should use the fisheye model, not a perspective model.

Are you saying that for the fisheye camera model, k4 is interpreted correctly? I'll fix it for you

elif camera.model == "OPENCV_FISHEYE":
    out["fl_x"] = float(camera_params[0])
    out["fl_y"] = float(camera_params[1])
    out["cx"] = float(camera_params[2])
    out["cy"] = float(camera_params[3])
    out["k1"] = float(camera_params[4])
    out["k2"] = float(camera_params[5])
    out["k3"] = float(camera_params[6])
    out["k4"] = float(camera_params[7])

I think k4 is correctly assigned as the fourth-order radial distortion coefficient and error check can be removed.