RAM is dependant on the application that you are running. But since you have 4 drives, I would suggest raid 1+0. That would give you 1 TB of disk space.
The RAID level that you select will depend on what kind of load you’ll be placing on the database. If you are using a low IO database application then either RAID 5 or RAID 0+1. If a higher write application then RAID 0+1.
This article covers the various RAID types.