I have thought about the length in between backups as far as lost data is concerned, The chance of losing all 3 other copies is very low and though 1 month could well be alot of work (depending on the month) it is still better losing 1 month than all of my files.
Keeping all 4 HDD synced is very easy.
2 are already in RAID.
1 is on auto monthly backup
the other portable passport has the
Geek.menu in it and has
allway sync and go in it as well, so everytime I connect and disconnect to the network I press sync.
Very easy to keep all synced
[Edit] When I travel I sync the portable passport via ftp, so if I lose the portable passport I still have my files