UE4 like any other engines is based on tradeoffs. It assumes a given minimum feature set on the platform, and makes full use of it. On the plus side, if you have met the requirements you can achieve more(or at least easier) than with an older engine, like Doom 3, could on the same hardware.
On the down side, you have the minimum threshold for your userbase that might be too high for what you are trying to achieve.
From the official minimum UE4 requirements for users:
Processor Quad-core Intel or AMD, 2.5 GHz or faster
Memory 8 GB RAM
Video Card NVIDIA GeForce 470 GTX or AMD Radeon 6870 HD series or higher
Windows 7/8 64-bit
(no Linux / OS X / SteamOS for now?)
What this means is, if you are making a game on the level of Half Life 1/SRB2/(insert any game from '90s) you certainly wont be able to run it on a machine bought before September 10, 2009.
My advice is to max out what you can do with UDK/Unity/C4/etc first and make sure it has a fallback settings that let the game run on older hardware so that most of the people here can actually play it.
I'm not trying to bash UE4, its a great engine at what its intended to do.But it also matters who and how is using it. It may save you some work and make some features easier to implement, but oh boy did I watch some "showcase" videos demonstrating GTA3 quality.