If I'm not intereted in keeping the data on my Terastation live, can I just repalce all 4 drives at once, instead of one at a time with re-build inbetweeen?
If I do this, do I need to burn firmware? Do I need to burn TPMS first?
Alternatively, can I copy the 4 current drives to 4 larger drives and plug the new larger drives in?
The first things you mentioned are correct but you do not need to burn TPMS first and you cannot copy the data onto the newer drives before putting the data in there as the data will be destroyed after rebuilding the raid array.