I have an WD EX4100 with two bays open; set to raid1 4x2 x raid1=4tb I want to add 2 more 4tb drives will raid rebuild or do I have to start over?
I'd not recommend you to keep the RAID 1 array with the WD My Cloud EX4100, since you'd be adding 2 more HDDs, @Wayne. With the NAS having 4 x 4 TB HDDs you can configure the storage with RAID 5 or RAID 10 even. These configurations would give you a much better performance and better storage capacity.
However, I'd not advise you to do a JBOD because you'd have no amalgamation, pooling or storage structure, which puts the data stored on the My Cloud at risk.
The backup procedure is a MUST when tempering with any storage, so there's no escaping this one! What you can do is backup your data from the RAID 1 array somewhere else and change the RAID mode from the Dashboard on your WD My Cloud EX4100. When you click on Storage, you'd be able to see the RAID profile. Once you have all 4 HDDs inside the NAS, you'd be given the option to change the RAID mode to 5 or 10.
The RAID 5 mode provides data redundancy, high read rates, and good performance in most environments. It also provides redundancy with the lowest loss of capacity. RAID 5 is best suited for scenarios where a lot of small input/output (I/O). With 4 x 4 TB, your RAID 5 array would have a total space of 12 TB available (and 4 TB for protection).
The RAID 10 mode combines RAID 0 and RAID 1, so it basically breaks up data into smaller blocks and mirrors the blocks to each RAID 1 drive group. The first RAID 1 drive in each group then duplicates its data to the second drive. RAID 10 is best used with data storage that needs 100% redundancy of mirrored drive groups and that needs the enhanced I/O performance of RAID 0. However, with the 4 x 4 TB HDDs, you'd have only 8 TB available (and the other 8 TB for protection).
Hope I was helpful. Let me know if you have more questions! :)