Res 5 is still a long way away. We're still actively developing Res 4 and there is enough still planned for it to justify a purchase now.
Resolume 5 will not be a free upgrade for Res 4 owners. But we're not some evil company that will let you buy something, then release a new version and expect you to pay the full price again. There are no fixed price plans yet, but you can expect the upgrade for Res 4 owners to be somewhere in the €100-€150 range, with a 'grace period' for people that just purchased prior to the announce of the release.
As always, it's good to be cautious when making big investments. But if you like Resolume and would like to start using it, you're wasting time finding other tools or interim solutions when you decide to hold off until Res 5.