RN102 and Active Directory domain

The RN102 is suitable for backing up the Pro - I use it that way in my home setup (though daily, not quarterly). If the backups are incremental, you probably won't be writing anywhere close to 4 TB. But I also don't think you'll get 40 Mbytes/sec average throughput - that is the max throughput for large files transfer. So backup times for a quarterly backup are a bit hard to gauge.

I'd also suggest that you do a scrub on the RN102 when the backup completes (or use some other tool to access all the files) - just to make sure there are no disk issues.

Other alternatives for disaster recovery include backing it up to a USB drive (for best performance connect the USB drive to a PC and back up over the network), and Cloud backup (Crashplan is what I use).

CrashPlan can be fairly easily installed on a pro, and is quite inexpensive. I get a backup speed of about 1.5 Mbyte/sec (more or less) though the de-duplication can make the effective speed faster than that. If you go with CrashPlan I suggest uplifting the stock memory on the Pro to at least 2 GB.