My 2c,
1) Not for me. 1080p is more prevalent in every way, and I'm not such a snowflake that I need my content to be "crisper" to enjoy it.
2) This is basically right, for me, but I feel also especially for Arma3. I don't even know if A3 runs at 4k?
3) In short, no. In long, you'd have to be really really really good at CSGO to feel the difference, again I don't think it matters for Arma3. I play on 4ms screens and always have.
4) I think so, but I'm not sure what the Arma support is like. I run 3x 1080p 24" screens.
5) Kind of. If the server is only running at 16fps, any performance you see over 16fps is not amazingly relevant. It might appear smoother to you, but the server can only register your bullets and movements 16 times a second.
Keep in mind, I don't believe performance in Arma is mega important. Arma is fun based on the gameplay, not based on the graphical quality, IMHO.