There is a requirement for Unity to package an Android 4.0 package. When I opened the Unity2018 version, I found that the minimum is 4.1 (API 16), as shown in the figure below:
So I thought of lowering the Unity version. I am currently using the Unity2018.4.9f1 version, so I found the Unity2017.13 version that was already installed. When I opened it, I found that the lowest version was still 4.1 (API 16), so I installed a Unity5.6.2 version and found that the package It was still 4.1 (API 16), so I installed a Unity5.1.0f1 version again, and finally I could package the Android 4.0 package. When packaging, I found an SDK error. The error was: Unable to list targrt platforms. Please make sure the android sdk path is correct. So I searched for the reason online and found the blog: https://blog.csdn.net/THmen/article/details/79424469 . I followed this blog and downloaded the old version according to the download address provided by the blog. After downloading the tools, unzip and find the tools inside, replace the tools in the original sdk, and back up the original ones before replacing them.
Finally, packaging is OK.