An IPS panel IMO is worth having, reproduction of colours and viewing angles are good.
A high refresh rate is not worth chasing if you are only going to use it fo flight sims, more beneficial to First person shooter games where you might actually achieve a high enough frame rate to utilise the screen refresh rate.
If you are only looking at 27” monitor I would be inclined to stick with a 2k (1440) screen, it is regarded as a good size/resolution combination- pixel density really. I think at that size for the difference in clarity you may see at 4K it might be better to save the processing power and aim for a better sim performance in terms of frame rate and smoothness. Or save money or sacrifice 4K in favour of other features.
Personally I would only consider the step to 4K if I were looking to get a 32” and above screen size, basically for the pixel density. Remember the clarity of a screen is more dependent on the pixel density and how close you are to the screen, than just a simple higher resolution must be better attitude.
Diminishing benefits of really high resolution (4K) screen the smaller the physical screen is (27” and below), but the larger you go 4K makes sense to achieve a higher pixel density that will become more apparent at larger screen sizes.