I'd also just like to add my vote to this.
I run a very large magazine website that relies on AdAgency to handle their banner advertising so if it's not developed for Joomla 4, that would be disappointing.
I'm guessing there's not much point then mentioning that you can't save campaigns under PHP7.4.
So hopefully the component is updated at some stage for both PHP compatibilty and for Joomla 4.
Thanks