Downloaded / installed newest v4 BI
Fresh Install windows 10 Pro x64
Dell Vostro 420
Core 2 Quad Core Q6600 2.4Ghz
Nvidia Gforce GT 635
8GB Memory
2 x 2mp/1080p Amcrest PoE cameras
2 x 8mp/4k Amcrest PoE cameras

I went though the list I found on the site here for optimizing performance e.g. direct to disk, 15fps (camera and BI) etc...
All 4 cameras are "always record" there are no alerts or motion detection options enabled.

If I set Hardware accelerated decode to NO , or Intel or Intell +VPP Blue Iris uses 100% CPU non-stop. The machine becomes so slow it's pretty much unusable but you can still access the cameras via the web interface (the stats at the bottom of the web interface show 100% CPU usage, 40% Memory, 5% HD , 4 FPS).

If I set Hardware accelerated decode to Nvidia Cuda then CPU usage runs around 85% -95% , the PC becomes acceptably responsive but still frequently pegs 100% while the GPU usage goes to and stays at about 50% and rarely spikes above 65%.

This was originally on a dual quad core 2.1Ghz Xeon 1U server that became completely unusable after the 2 x 4K amcrest cameras were added (same config, direct to disk, 15fps, always record, no motion/alerts). While again, the BI web interface was accessible the server itself literally took hours to log into (and BI was all it was running) AND there were large, random gaps (a few minutes to 30+ minutes) in the recording. So deactivated the server and did an all fresh install on this admittedly older Dell thinking the ability to install/Use the Nvidia GPU would resolve the problem and if I needed a more powerful GPU than the old GT 635 then I could install it in the Vostro where any GPU at all was not an option on the 1U Supermicro server.

So I'm wanting to add two more 4k cameras and it's looking like the GPU can easily handle it but the CPU usage is already way more than it seems it should be and it's not going to handle 2 more 4k cameras.

So should I expect this server to handle these cameras using a GPU or do I need a more modern / newer gen CPU regardless of using GPU to decode ?

Not sure you'll get too much help, being you installed v4 (newest version!). Did you typo v5? And what version exactly? Newest version changes.

It's not free, but I highly recommend the upgrade to v5 if you can do it. There are so many useful improvements and enhancements in v5 that it's absolutely worth it in my opinion.