2 x RAID1 vs. RAID10 with four disks

RAID 10 is just stripes over mirrors. That means for every mirror pair only one drive may fail. You also lose 50% of the storage. I don't know how your NAS handles two RAID 1 arrays but it might be a bit faster to use RAID 10.

If you want better fault tolerance you could use RAID 6. There you still have 50% capacity loss but any two drives may fail.

RAID 10 is probably the fastest and RAID 6 the most secure.

RAID 10 differs slightly from 2x RAID 1: it stripes the two mirrored subarrays, improving the performance, but it relies on availability of both subarrays. In a nutshell, RAID10 will fail totally when any subarray fails completely (both disks).

With dual RAID 1, the second array will stay online even if both disks in the first array fail.

If you require the array to survive failures of any two disks you need RAID 6 as Christopher has already pointed out. Due to the necessary computing overhead and the performance of the NAS's Marvel CPU, you should expect a significant performance hit on writes though.