It should be expected that most games which got full-on 4k res from 1080p would end up with a similar performance profile.I'd chalk it up as a fluke, but I do think the OP should be updated with "minor dips."
4 times the power. 4 times the pixels. Should be about the same performance (even if X1X may be slightly more powerful overall than 4 times due to GDDR5 and other optimizations).