2 of 4 Drive Failure-- what now?

Using Raid 10 and so glad I am because I just logged into my EX4 for it to report that 2 drives are bad.

Currently using 4 3TB drives for a 6TB total storage. Using all, but 800 GB at this time.

From what I understand I can simply purchase 2 new 3TB drives and replace the bad ones and the NAS will automatically handle the rest, but if I am going to purchase new drives anyways why not get a larger capacity drives and replace all 4?

Question is what is the best way to do this? Buy a large 6TB external hard drive and back up the data from NAS to that? Can I avoid this somehow?

Thanks,

Jeff