As briefly discussed on Discord, I have a little idea to revive some members after the launch of V4.
After its launch, or just before, why not inform all members of it by email? Highlight the new features, maybe even entice them with a cheeky but meaningless award?
Naturally they would probably log in to get the award, but let's hope the new features/design will encourage them to stay.
Or instead of such a simple award, maybe even referring an X amount of members to V4 could allow for an award to make it a bit more difficult while at the same time getting more members.