Can two Macs share one Time Machine backup drive?

Yes. But it takes a little more setup on the sharING computer:

Back up to a shared folder with Time Machine on Mac - Apple Support

My question: Can I share the 6 TB Time Machine drive from the M4 Pro so the 2014 mini can also back up to it without partitioning, and will Time Machine automatically keep each Mac’s backups separate and safe?

Yes.

I should probably bifurcate that question.

My question: Can I share the 6 TB Time Machine drive from the M4 Pro so the 2014 mini can also back up to it without partitioning, and will Time Machine automatically keep each Mac’s backups separate ...

The answer to that question is yes.

... and safe?

"Safe" connotes different things to different people. If it's not safe I don't do it, and I don't do it.

Vernon,

Using the same Time Machine backup External Hard Disk for 2 machines is a bad idea, why very simple read on.

All EHD's eventually fail and will lose all the data on them. When that occurs on a Time Machine External Hard Disk the data is lost, so why would you do that on 2 Macs??? EHD's are inexpensive these days and there is NO reason to scrimp on them.

Wise users have redundant backup plans on their Macs. For close to 20 years I have used 2 EHD's connected to each of my Macs, one does a Time Machine backup and the other creates a clone. The clone drive can use a paid for (relatively inexpensive) app such as Carbon Copy Cloner or SuperDuper!.

I can not comment Time Machine, but I have used the same APFS drive to make bootable backup volumes with Carbon Copy Cloner. Those old Macs have different macOS versions ranging from High Sierra to Big Sur and Ventura and each one uses the last compatible CCC version). But I have used this only for my old non-mission critical Macs so when those old Macs' disks break, I can put in another disk and quickly clone a bootable backup to that instead reinstalling everything from scratch.

For the main Mac running Sequoia I use a dedicated 1TB APFS drive to make a bootable clone with CCC (it does not have to be bootable but old habits are hard to break. Maintaining bootability with the macOS updates occasionally takes some extra care that might very well be wasted time...).

That main Mac is also backupped to another 2TB APFS drive that additionally holds a little bootable Sequoia test volume (to test 3rd party apps before installing to the main internal drive) as well as a data volume for Photos library (which is backupped elsewhere).

+1