Of this morning (26th of March 2020) the R 4.0.0 build is fixed and you can download the R-devel tar.gz from https://mac.r-project.org/
For the installation, I recommend following along the guide from RSwitch (https://rud.is/rswitch/guide/index.html).
- Download the latest R-devel build:
wget https://mac.r-project.org/el-capitan/R-devel/R-devel-el-capitan-sa-x86_64.tar.gz
- Unpack it at the root of your file system
sudo tar fvxz R-devel-el-capitan-sa-x86_64.tar.gz -C /
- Open a new terminal and check if the installation was successful
~$ R --version
R Under development (unstable) (2020-03-25 r78063) -- "Unsuffered Consequences"
Copyright (C) 2020 The R Foundation for Statistical Computing
Platform: x86_64-apple-darwin15.6.0 (64-bit)
The nickname "Unsuffered Consequences" indicates that the installation was succesful.