It's just a recommendation. Remember, some people use FreeNAS professionally, so they're going to err on the side of recommending too much (= enterprise). Less will probably work fine, as long as you aren't running dedupe, compression, or heaps of jails and services. E.g. if you have 4 drives and just use the SMB service to connect to one or two machines, 4GiB is probably okay.
Having said that, RAM is pretty cheap. 8GiB isn't a huge ask for any machine in 2016. Any system with less than 4GiB, is it really worth bothering with redundancy/ZFS? UFS (which is far less RAM hungry) + nightly backups might be good enough. (Edit: of course, you miss out on the awesome ZFS features with UFS. It's never easy, is it?)
Having said that, RAM is pretty cheap. 8GiB isn't a huge ask for any machine in 2016. Any system with less than 4GiB, is it really worth bothering with redundancy/ZFS? UFS (which is far less RAM hungry) + nightly backups might be good enough. (Edit: of course, you miss out on the awesome ZFS features with UFS. It's never easy, is it?)