Without a backup, you need a temporary, identical NAS device with the 4 new disks. Then do a file-level copy. Then put the 4 new disks in the old NAS and import the array. Do you know of a store with a very lenient returns policy?

An online capacity expansion (replacing one at a time) will mean a very long time in a degraded state, possible multiple disk failures, increased chances of data loss (power goes off, etc)