From a performance standpoint, logical striping across four LUNs will give very good performance for your backups. In the event of a disk failure on one of the RAID sets, you will see some performance degradation – just how much depends on how long it takes the array to rebuild, I/O load, etc.
As along as your software supports volumes of that size, you might as well stripe across the lot of them. By my math, if you’re doing a total of four RAID5 sets with no hotspare, each RAID set is almost 1.8TB or ovre 7TB for the lot. Be sure your OS supports a logical disk of that size, before you stripe them.