Richard, you’ll be fine. Pix4D will identify the camera and use the standard parameters, but you can make a rough edit of the camera model to reflect the change to the lens. Afterwards Pix4D will by default refine the camera model and calculate a much more exact focal length, along with optimizing other lens parameters.
From the image properties menu you can edit the camera model, saving the optimized parameters for the next time. You’re self calibrating the system to an extraordinary degree.
I’ve used the stock lens in a GoPro, and I’ve used a longer focal length as well. The GoPro isn’t my favorite but they are expedient, have a lot of mounting solutions, and solve the problem of finding a camera that is lightweight with a fast enough intervalometer.