Best Way to Consolidate 4 NAS Servers down to 2 new NAS devices without buying new HD's

It's hard to answer this completely, as you have not said what drive sizes are in which NASes, how full they are, nor the use of each existing NAS (primary or backup).  But I will comment on some of the implementation plan.

Personally, I would have upgraded the processor and memory on the Pro BE, installed OS6 on it, and bought bigger drives for it instead of spending the money on the new NASes (that is what I did instead of going for a new NAS).  But if this is a production environment, you may be wanting to eliminate the older units proactively before they start having problems, you may need to have more future expansion capability, or you may need the 10GB Ethernet or other performance enhanements of the newer NASes.

If everything is backed up between all these devices, then the risk is reduced that you'll lose eveything if a drive fails after you have intentionally made all the arays non-redundant.  But it still has risks.  If anything isn't backed up, this is highly risky.

I recommend you use NAS backup jobs instead of dragging and dropping via Windows,  Putting a Windows machine in the middle results in a "bucket brigade" instead of a direct transfer.

I use Acronis True Image for my PC backups to the NAS and a built-in Rsync NAS-to-NAS backup for backing up NAS content.  For offsite NAS-to-NAS backup, I still use the built-in Rsync and use ZeroTier for the VPN between them.  Since I keep critical data on the NAS, I don't do an offsite backup of PC's.  There are some decent open source solutions, but Aronis is worth the money to me.

Crashplan can do mapped drives, though the implementation may not meet your needs.  See Backing_Up_Files_From_A_Windows_Network_Drive.  I believe that another option is to create a junction (aka hard link) to the NAS folder instead of a mapped drive.