I'm not using de-duplication, so that might affect if you only have 4GB RAM. Right now the box has 2GB free, and I'm always running a few extra things in it. Overall, for my use case it works just fine. The SATA card is plenty fast as well. I had a similar setup with an rpi3 and a USB<>SATA to host 2x1TB disks and doing a scrub in that pool took over 4 hours. The scrub in my current pool (4x4TB) took 22 min.