Hi Stefan,
A) Yes it is/was possible
B) Yes, on the extra modules. NO, on the redundancy and availability.
C) The Parts you are looking for are all obsolete and best chance is Ebay or another 2ndhand store.
Depending on how you would like to stack them.
For the 4000M look for:
J4113A Gigabit SX fiber SC-connector
or
J4114A Gibabit LX fiber SC-connector
or
J4115B Gigabit Copper RJ45 Cat5
For the 2424M
You need 2 Stacking modules J4130A (1 for each 2424M
and
J4113A/B Gigabit SX fiber transceiver
or
J4114A/B Gigabit LX fiber transceiver
or
J4115B Gigabit Copper transceiver
So depending on how you want to stack and if you want copper or fiber then the amount of transceivers and modules are depending on that.
Best Regards,
SCOOTER