Here is a small guide by Cristal on how to get the Aion RU 4.0 client and log in to test out 4.0! Credits: Cristal
login with facebook
on the bottom there is a launcher ~4mb, install it
then you need a bookmark with this as url
on aion website click on that bookmark and you should see a small window on the left with QA, PTS, PTS2, PTS3, LIVE
chose any pts
and then click yellow button to start downloading
You now know how to download the client.
Not sure if login is straight forward or not, but I'll update the guide if it isn't.
PS: Make sure it says Aion [PTS] on the 4game page, or else you'll probably download the live version. >.< I made that mistake but was only like few minutes in.