So this is our first QNAP,
we just purchase it, waiting to arrive.
we just purchase 2 HDD first because our data is not that big yet,
but we plan to add another 2 HDD if the space is not enough.
Which Raid to choose?
Or we should choose to buy 2 more HDD?
Personally with a 4 bay NAS I would go with 4 disks and use RAID5. It just saves expanding the raid and hard disks at a later date.
Remember your NAS will require a backup solution too. whether it be to cloud/USB or another NAS. Different Raid types give different fault tolerance. if you loose the raid altogether you have at least the backup to restore from.
Also with a NAS using raid you will need a UPS to protect against power outages. the raid can be damaged with power being cut abruptly. a UPS allows the NAS to carry on being powered for a number of minutes before the power comes back or it will shut down the NAS gracefully and protect the RAID.