I have the same issue: 4 Macs in the house, and I want each of them to use Time Machine to back up to my new (OS5) PR4100. And I want each of those backups done on a share of its own … that is, one share per Mac.
But the PR4100’s admin software seems flawed: I can enable share “A” as a TM destination, for use by Mac A, and get its TM backups fully working. Then I go to the admin page of the PR4100 and set a max backup size for share B, to enable that share. I set up Time Machine on Mac B to use share B, and it all seems great … EXCEPT that (maybe after a few hours) Mac A can no longer do backups to share A any more - it fails with an error message on Mac A saying that its Time Machine disk (share A) does not have the necessary capabilities! (Grrrrr!).
From there … if I go back to the PR4100’s admin page and (again) set the max size to use for TimeMachine on share A … well, Mac A can again back up to it - resuming use of the backup file it had started there. BUT (in true whack-a-mole style), that has disabled TM access to share B, and made share A the ONLY one usable for backups.
I saw a note on some discussion forum that said the trick was to be sure to let each Mac get all the way to having completed its first full TM backup (the slow one, which can take days) - and that then it would be safe to tell the PR4100 to enable TM on a different share, and get things going with that one … one Mac at a time. However, I tried that, in various combinations & variations for several weeks … and could never convince the PR4100 to keep more than one share at a time operable as a destination for TM backups.
I also took a look “under the covers”, using ssh to login to the PR4100 and look at the relevant configuration file. Sure enough, the file that specifies the set of shares, with a block of details for each share, gets an extra set of details (about 6-10 lines worth) for whichever share is the most recent one to have been enabled as a TM destination. And using the PR4100 web admin console to enable a different share adds the desired set of config options to that share, but (and here’s the BUG) … it, at the same time, REMOVES the lines that enable time machine access from all other shares.
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tl;dr: Enabling any given share for use as a destination for Time Machine backups actively REMOVES the enabling configuration details for all OTHER shares.
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So far, the only work-around I’ve found is to have all of my Macs use the SAME share on the PR4100 for their Time Machine backups. There’s no collision (assuming, I guess, that each Mac has a different host name), but it’s not ideal, either administratively or from a security “best practices” perspective: If one Mac were to be compromised (say, by a ransomware attack), the attackers would have access to read/destroy the backups for ALL of my Macs. 
