Essentially the ISP will be trying to lease the same IP to 2 devices at once.

But you only have one connection from your ISP, so I don’t see how this is possible.

Any desktop computer that has enough SATA ports and space in the case for your desired configuration, that is from this last decade, should work fine.

Dell rackmount servers are power hungry and loud.

TLDR:is a 4 Bay chassis provide sufficient protection for freenas ZFS with total ~ 10TB usable storage? or do I need 8 Bay?

There is no requirement of a particular number of disks for ZFS. You can run ZFS on a single disk.

But for TRUENAS (FreeNAS name has been dead for years now), you will dedicate one entire disk to the OS, so keep it small (just shove a small SSD in the case somewhere), and then decide how many disks you need to buy to accommodate the amount of storage you need. Then decide if you ever want to expand that capacity. THAT should determine what kind of case you buy.

Is a dell R320 or R510 sensible in terms of power efficiency for freenas?

Dear God, no.