Jack wrote

[...]

Oyster cannot retrospectively apply a Z1-4 cap when a Z5 journey has
already been made earlier in the day

This seems to be entirely your invention, not anything official, the
standard view being that all caps are applied all the time so
suggesting retroactive fare setting is needed is confused.

Oyster axioms include "no negative fares" and/or "order doesn't matter"
but that not the same thing.

My files produced

========= From: Mr Thant 02 Dec 2009
is that each journey you pay for contributes to all of the possible
caps that might apply, and if any of those caps is reached you only
pay the difference (or zero).
{lots more}
==

And no one on uktl from that day to this has queried that all possible
caps can apply all the time so a Z1-4 cap is quite independent of any
Z5 or Z6 journeys that may have been taken.

--
Mike D