Given Trans Pennine are procuring new loco hauled stock when they could have taken some of the Mk 4 s, that's probably a clue as to how attractive they are to potential operators.
I thought the reason
TPE▸ ordered the mark 5a coaches was they needed some new stock quickly and couldn't wait for new-build multiple units. According to Wikipedia the mark 5a coaches are due in 2018; mark 4s are not due off-lease until 2020 so it could be that mark 4s weren't an option simply because they would not be available until too late.
As for the rest of the fleet...Thoughts mentioned have been London to Norwich (now brand new IC▸ fleet) and London-Sheffield (now bi-mode)
Neither of those routes made sense for IC225; without tilt the only sensible alternative to the
ECML▸ for the 140mph class 91s was probably the
GWML▸ . The mark 4 coaches (without the 91s) could have been useful on the Norwich route or Midland Main Line (
MML» ). In fact, if somebody can come up with an
ETS▸ (Electric Train Supply) adpater for class 43s to allow them to work with mark 4s, I think using them on the MML could still be a good option. That would allow them to run on through
CP6▸ (ScotRail are keeping IC125s going at least that long) and thus give a chance of MML electrification in CP7. Such a shame the
DfT» want bi-modes to lock the MML into diesel for another 30 years; that route should have been wired before I was born (
BR▸ 's 1981 electrification plan put the MML before Newcastle to Edinburgh).