There is a lot that needs to be considered. definitely 4k is better, but for gaming a single 290x will not be able to run all games smoothly even at mid settings.
1440p is detailed enough for most people, and coming from 1080p definitely it is a big step.
Personally coming from 1080p to 1440p to 4k gaming, definitely there is a big jump from each step with a huge demand of GPU power in each step.
The monitor will have to fit the GPU power available... I would suggest 1440p for your setup.
good luck.