imo buy the better GPU if possible, and RAM.
I use a 4-year-old laptop with 6GB RAM and a ATI 7670M GPU, no SSD just a 5400 rpm HDD, i5 CPU (2 cores +2 by hyperthreading), and I can work with UE4 thanks to scalability settings (but 90% I work with Visual Studio, only 10% editor), but sometimes very slooooow.
Usually I have a GPU bottleneck when testing my game, that can turn into a CPU bottleneck when many moving actors are present, but I work on a computation intensive RTS system. Luckily, it is a great motivation for performance optimization at least 
(which is important because RTS players do not own the best gamer pc-s usually).