There are two main places to download previous versions or nightly builds of 4D.

Public Releases:

Public releases are accessible to all users and can be downloaded from the main site link below:
https://us.4d.com/archives

Public Releases + Nightly Builds (Partner Only):

For specific builds between v17 and v20, these can be found from the partner download page in the link below:
https://product-download.4d.com/