CPU is more useful for editor then GPU, primery because you can always turn off real time rendering, memory is also super impotent, if you have 8GB upgrade definitely should speed up UE4. So if you want to wait for something, GPU is the one you can wait on.
Also note that more core = better for editor, as shader compiler will work on all cores also more cores is better multitasking capabilities of you PC. So… you might want to wait what AMD has to offer with Ryzen 3xxx as rumors are quite interesting, as they might be very nice CPUs for game development for great price.