I had a similar issue just now. All of my buddypress pages (register, activate, members, etc.) were giving me a 404 error. I now have all the pages displaying correctly.
EXPERIENCE
I'm using an amazon EC2 instance, with apache2. My buddypress was version 2.2.1 on wordpress 4.1.1.
My first guess was that the URL wasn't rewriting correctly, so I tried all of the options in settings > permalinks.
Each time I reloaded the page I was trying to access and confirmed that my .htaccess file was being rewritten accordingly.
(.htaccess is in the base directory of your wordpress installation. If it isn't being rewritten by saving wordpress' permalinks options, try chmod-ing the permissions: http://codex.wordpress.org/Changing_File_Permissions).
SOLUTION
I found a solution through the permalinks page of the wordpress codex:
http://codex.wordpress.org/Using_Permalinks
There is an option is a file called httpd.conf on my system that disabled the rewrite module in my .htaccess file. (On a linux system this should be in /etc/httpd/conf. On OSX it should be in /etc/apache2.)
By changing my webserver directory AllowOverride option from None to All, the .htaccess file was able to function properly. Again, I found a line in the section of my httpd.conf file that read:
AllowOverride None
and changed it to:
AllowOverride All
Hope this helps.