Just fill the QT tank 24 hours prior arrival and let the filter and heater run .
Fishless cycle or quick start bacteria cycling would have been safer to rule out - cross contamination , but you are already set and ready with the display cycled sponge .
What's the QT and display tank volume ?
What other fish do you plan to keep with discus ?
Why only 4 discus when 6 is considered the safe minimum ?