To get OpenACS 4 read this: [1]
https://openacs.org/new-file-storage/one-file?file_id=114It explains how to check out the latest OpenACS 4 source. OpenACS 4 is not finished yet though. You can download OpenACS 3.2.5 [2].
https://openacs.org/sdm/one-package-release.tcl?release_id=21
Also read the porting wimpypoint presentation [3] before attempting to work on OpenACS 4. It has a lot of valuable information.
Good luck.
[1] https://openacs.org/new-file-storage/one-file?file_id=114
[2] https://openacs.org/sdm/one-package-release.tcl?release_id=21
[3] https://openacs.org/wp/display/10/