Hi,
yes, install Veeam in a VM in the ESxi n. 4, so it's outside of the production servers.
About RAID, raid6 gives you a double parity so an added protection, but the write penalty is almost doubled, so performances will be lower than raid5. Do some tests and see if the performances are enough for you.
About licenses, Veeam requires licenses only on source hosts, not the target ones. If you need to run all from n° 4, you would be able to revoke licenses from first 3 servers and move it the the fourth.
Luca.