I used to run UE4 in a i5 25K, 8 gig ram, ATI HD Radeon 6790 Saphire and it worked fine. Only a few lag in the effect cave demo.
I now run UE4 in a i5 25K, 8 gig ram, XFX HD Radeon 6870 Black Edition(One card only) and it runs flawless. That is a card from 2011!!! I can even have the Cave demo open and my own project at the same time. I do see some minor crashes(after like constant use of 8 hours plus) but mostly due to driver issues or playing with the fbx animations.
You won’t need a huge rig with 16 gig plus ram. Older cards are fine and with an i5 or i7(or equivalent amd cpu) you should be fine. Of course I am not making an MMO with huge open worlds 
It all depends what you are making I guess.