There is no official guide at the moment on their website, however there is a basic teardown video iFixit made on YouTube, however it is slightly brief. Jerryrigeverything just released his teardown video on YouTube, along with a couple of other videos on YouTube. He said it is similar to the Pixel 3a - however getting a proper guide before repair would be preferable. Taylor from iFixit talks about the Pixel 4a here; https://www.ifixit.com/News/43537/the-pixel-4a-is-actually-new-and-interesting-once-you-open-it
Hope this helps