Backup strategies are like lasagne recipes; everybody has their own, and fights can get started depending on who’s in the kitchen on Sunday morning.
My longtime issue with creating two partitions on one Drive for backups is that both end up requiring/using unused disk space, and learning you’re staring at hundreds of GB of wasted space down the road is frustrating. Deciding to resize later can often be a big pita, if you don’t have yet another big fat drive to p,at swap-it.
Saying that you have a 1TB System Drive, and presuming you should thus have a matching (or greater) clone volume ignores actual current and future disk use, not to mention archiving features for the clones.
If you only have 129GB of System and data on your Mac, why burn a 1TB external volume for a clone (unless you want a huge depth of archives, especially for very large files which change minimally)?
Time Machine, likewise, tends to go into “Sorry, Dave, I’ve perceived an undescribed error I cannot repair, and I’m unwilling to let you repair it, either, so I’m leaving you no option but to nuke your last two years of carefully curated incremental backups and start from scratch.” -Mode the more often it bumps into a full disk and is forced to prune older backups to make room.
Given the $ choice, it’s one drive for clones (and temp manual backups/storage/data transfer games) and one for Time Machine. But to this day, I have no problems whatsoever allowing TM and CCC to share a single volume/drive, with storage limits and generous pruning settings for CCC, and, of course, protection set for root items. And I skip backing up most System/app data via TM, because it’s covered perfectly by CCC; I’m only interested in its hourly, incremental, easy to use user data restoration features.
If you do decide to split one drive for TM/Clones, be sure to put TM on the first partition, despite being faster and a normally a better choice for the clone (in case you need to emergency boot, a clone first is way faster to boot/use), because it’s far, far easier to resize a drive if you don’t have to erase/restore the TM volume.
To that end, if you can afford to leave the existing too-full TM HDD alone and just preserve it as an archive, and it’s easy enough to just pull out to access that old data if required, I’d advise starting a new TM backup from scratch, for two big reasons: one is the aforementioned “screw you, Dave” issue; it’s nice to know you can always drop back several months/years, despite random TM farts; two, it can take DAYS to transfer a TM volume to a new one, depending on interface speed and drive speed, and once you start, you’re committed; no restarts or interruptions allowed, lest you begin again — and no TM backups can occur in the meantime.
Once you’re confident that original archive is no longer needed, you can then repurpose that drive; indeed, it sounds like a nice size match for cloning, to begin with. In that case, why not devote the new 4TB to TM, with a temporary clone volume racked on the end; run that for a short term; then nuke the old TM, make it your SuperDuper drive, and zap the temp clone partition and let it fill out as a deep TM. Or continue with two clones and keep one offsite, etc.
We generally have at least two lasagna here, sometimes three, depending on the veggies in attendance.