I definitely agree with Windies: Arma 4 doesn't need a new engine, but RV needs to be developed and enhanced so that there are less instances of "hacky" code that negatively impacts the long-term future of the engine.
In addition, there are still performance issues remaining to be solved, nevermind all the improvements they could make in terms of animation, view distance, and visual effects. Probably the most important of these is true 64-bit support and proper utilisation of multi-core CPUs. Both of these qualities will define good engines by the time 2018/2019 rolls around, and not having them would severely hamper Arma 4 as compared to other games given that Arma is already more demanding than most series.