实测ubuntu18 没有办法在树莓派4 8GB运行，原因不明。
会出现如下错误：
recover4.elf not found
recovery.elf not found
start4.elf not found
安装ubuntu20可以正常进入系统
所以这就成了树莓派4 8GB版本安装ubuntu20教程
首先换软件源（清华源）
deb http://mirrors.aliyun.com/ubuntu-ports focal main restricteddeb http://mirrors.aliyun.com/ubuntu-ports focal-updates main restricteddeb http://mirrors.aliyun.com/ubuntu-ports focal universedeb http://mirrors.aliyun.com/ubuntu-ports focal-updates universedeb http://mirrors.aliyun.com/ubuntu-ports focal multiversedeb http://mirrors.aliyun.com/ubuntu-ports focal-updates multiversedeb http://mirrors.aliyun.com/ubuntu-ports focal-backports main restricted universe multiversedeb http://mirrors.aliyun.com/ubuntu-ports focal-security main restricteddeb http://mirrors.aliyun.com/ubuntu-ports focal-security universedeb http://mirrors.aliyun.com/ubuntu-ports focal-security multiversedeb-src http://mirrors.aliyun.com/ubuntu-ports focal main restricteddeb-src http://mirrors.aliyun.com/ubuntu-ports focal-updates main restricteddeb-src http://mirrors.aliyun.com/ubuntu-ports focal universedeb-src http://mirrors.aliyun.com/ubuntu-ports focal-updates universedeb-src http://mirrors.aliyun.com/ubuntu-ports focal multiversedeb-src http://mirrors.aliyun.com/ubuntu-ports focal-updates multiversedeb-src http://mirrors.aliyun.com/ubuntu-ports focal-backports main restricted universe multiversedeb-src http://mirrors.aliyun.com/ubuntu-ports focal-security main restricteddeb-src http://mirrors.aliyun.com/ubuntu-ports focal-security universedeb-src http://mirrors.aliyun.com/ubuntu-ports focal-security multiverse