First the ugprade to 8 isn't optional. That will bring up up to "minimum specs" for FreeNAS.
Second, if you plan to run plex for transcoding you are going to be woefully disappointed with your server's performance assuming it can even keep up with transcoding.
Third, the 2.2TB limit is going to be totally dependent on your hardware chip, its revision, BIOS version, etc. That's not a question for this forum, nor should it be answered by this forum, nor did you even provide enough information to actually answer the question anyway.
Fourth, if you plan to run Plex in a jail, you're going to need more than 8GB of RAM. You need to have 8GB of RAM plus whatever RAM you are going to need for jails on top of it. I can tell you that my plex server uses 1GB+ when nobody is using it, and can often hit 2GB of use with just 1 user.
Fifth, you may find that your hardware won't even boot FreeNAS. Often Dell/HP/etc isn't even compatible with FreeBSD and won't even boot.
tl;dr - Your hardware is inadequate for this task. You're going to need to look at something bigger if you want to use FreeNAS. You may