Yeah, 4k+
You don't need more passes with higher resolution, quite oppositely. Noise becomes less perceptible with higher resolution. Just like with real cameras. Noisy high-res image from DSLR still looks good. Noisy low-res shit from cellphone does not.
100 passes for 2K, 4K or 8K should give you the same noise level roughly, but 8K will look good sooner, it may already look good with only 50 passes.