Drobo FS won’t add 4TB as a replacement

I have 2x Drobo FS, both with 5x 2TB HDDs. Been running fine for years, w/ the occasional drive replacement.

My Drobo Dashboards are current on my Macs as well as the most updated FW.

One of them was running low on space and I found out that the FS could support 4TB. I purchased a 4TB WD Red and replaced the one blinking yellow HDD. The 4TB seemed to be recognized (per Dashboard) and started to data protection process. When it completed, the 4TB HDD showed up as red. Not sure what happened, I have put the 2TB back and it’s rebuilding back to where I was.

WD Red appears to be a good drive. It’s new and I formatted and tested it on a Mac.

Any ideas what’s going on? Do I need a pair of 4TB to make this work? Or do I need to replace a good, green drive with 4TB, before replacing this yellow drive?

Short of going to a new NAS w/ much larger drives, I’m at a loss here. Any help would be much appreciated.