If you participated in any of the other betas, you just log in to your account like normal.
If you have not, I believe tomorrow you can go here EDITED WITH UPDATED LINK:Â
http://entry.ffxiv.com/beta_regist/na
and click the link at the bottom that says Proceed to the beta test application site.
The site is closed right now, but says it will open when beta 4 officially starts (tomorrow). Â You should be able to sign up there to gain access.
If you have not, I believe tomorrow you can go here EDITED WITH UPDATED LINK:Â
http://entry.ffxiv.com/beta_regist/na
and click the link at the bottom that says Proceed to the beta test application site.
The site is closed right now, but says it will open when beta 4 officially starts (tomorrow). Â You should be able to sign up there to gain access.