Fallout 4 uses the same game engine as Skyrim. These games are above average cpu usage. Also, because of limited thread usage, the engine relies very heavily on 2 things. Core speed and IPC.
FX Cpus have an IPC about 67% performance vrs Intel. 2x threads of an FX cpu need to be running @4.5GHz to equal a 3.2GHz Intel cpu. Roughly. To roughly get the same performance of a 4.0GHz Intel, an Fx cpu would need over 6.0GHz. Or, going the other way, a little g2020 pentium would give an Fx8320 a run for its money on either of those games.
I'm not knocking amd, they do have their good points and where they are strong, just happens not to be in gaming, especially on high cpu, high core speed, high IPC games like Fallout 4.
Games like Battlefield 4 are different. Those engines rely more on core count than IPC. An 8350 with decent OC tops everything but (prior to skylake cpus) the i7-4790k and 2011(-3) builds, surpassing even the i5-4690k, simply due to core count and usage.
Zen is supposedly a raise in IPC of @40%. This should bring the FX series upto Haswell levels in IPC. Or thereabouts. If it works as planned, Zen will put amd back in the running, still second place, but a damn sight closer race than the present.