I also installed Unraid last night and it worked flawlessly, so I do think that Freenas doesn't agree with my network card.
I think you are spot-on with that. FreeBSD is a bit strict when it comes to hardware support, not nearly as loose as Linux. Also, the "8 GiB minimum, 16 GiB to get something decent out of it; three disks can get you somewhere, six is great" guidance for FreeNAS is real. ZFS trades RAM for speed, and relies on multiple physical disks to do its checksumming work. ZFS with one drive doesn't give you any advantage over any other file system.
In my mind, the main reason to choose FreeNAS is ZFS. With one drive and 4GiB, FreeNAS is a decidedly sub-optimal choice.
Your options as I see them are:
- Stick with UnRaid. Nothing wrong with that, particularly with one drive.
- Go for FreeNAS and buy the extra hardware to make it happen: A second drive to mirror the first one (does the enclosure have room / SATA ports for that?), an SSD boot drive if you can, USB 2.0 (!) stick if you can't - main concern is USB dying on you; more RAM to get to at least 8 GiB, better 16 GiB, and an Intel add-in NIC - can the motherboard / case handle that?
The second option might be more hassle than it's worth. That's up for you to determine.
Edit: Before you make a decision, read
https://forums.plex.tv/t/freenas-vs-unraid/169684/6 and consider how important data integrity is to you. With a single disk, there is none either on UnRaid or ZFS - with multiple disks, ZFS has the advantage, though that comes at the cost of less flexibility with regards to hardware choice and disk layout.