Adobe CC downgrade back to CS 4 - duplicate locked

I'm trying to downgrade my Adobe design/photo software back to CS 4, which I have on CD. I have cancelled my CC subscription.

I've been talking to Adobe Support over the last 24 hours. They can't get it to install. I get major installation errors and trouble with Adobe Cleaner Tool, which just freezes—both installing CS 4 from my CD and from the Adobe software link they provide.

Is Adobe CS 4 not compatible with Yosemite 10.10.5 after all? I had researched all of this and had read it was compatible. There is some kind of conflict—or leftover Adobe CC data in the system?—that prevents me from installing it on the same computer and OS that it was on before I had to upgrade to CC a few years back.

Something in the CC upgrade made my CS 4 deactivated and from what I've been reading from other Adobe CC users, it was intentional on the part of Adobe.

I just saw a post that talks about using an external hard drive and booting an older OS on that. https://discussions.apple.com/docs/DOC-10421

Any help is appreciated! Thank you.