Expanding a FreeNAS RAIDZ Pool

I am building a ZFS data server using FreeNAS. I have enough hard disk space (4x2TB) for a few years, but if down the road I want to expand this out can i build another 4xHD pool? Would i need separate datasets for this second volume or can i just expand the existing ones?

You have various options. I'm assuming here that you have the 4 disks arranged as a raidz1 (like raid5) layout. Not ideal, but I won't go into discussions about the best disk layout.

1. You can go and buy big 4/5/8/XTB disks in a few years and just replace the disks one by one. When they have all been replaced the size of the pool should increase.
2. You can add a second group of 4 disks to the original pool. (Adding a new vdev in ZFS terms). The new vdev should ideally be the same type as the first, so if you first 4 disks are set as raidz1, the next four should be as well.
3. You can make a new, independent pool out of the new disks.

Yes. You can swap out the disks in a RAIDZ. You can't change the number of disks. You can also add another RAIDZ to a pool, but you can't remove a RAIDZ from a pool. You might want to take a look at the ZFS Administration Guide before deploying it.