That's going to be kinda tough. On the one hand, 970 SLI plays 4k surprisingly well, but I don't recommend it because you are limited to 3.5GB VRAM, which is already not enough to use max textures in some games.
A single 1080 Ti will play most games at 4k with 60 FPS, but it's at least 850 Euros in Europe from what I'm reading, which would make it very hard if not impossible to fit your budget.
It really depends what you call "good frame rate", and what games you play.