What are the parts of your current pc that you want to upgrade?
What will be the main use for this build?
If it is for multithreaded batch applications, then amd in either the fx line or ryzen gives you many threads at a good price.
If your use is for gaming, realize that few games can make effective use of more than 2-3 threads and the single thread performance of the master thread is all important.
Here is a nice link to motherboard tiers and what processors are suitable for each:
http://www.tomshardware.com/forum/id-2383150/motherboard-tier-list.html
If you are buying new, I would not buy the FX line. They are all but obsolete.
Used might be another matter since used prices reflect price/performance.
You can get an approximate comparison of processors by looking up the passmark performance numbers.
For example the 6 thread FX-6300 has a passmark rating of 6384 when all threads are fully utilized, and a single thread rating of 1408.
The 4th gen 4 thread G4560 you mentioned has a rating of 4833 and a single thread rating of 1908.
The 8th gen intel G5400 is an excellent budget chip with 4 threads and a passmark rating of 5251 and a single thread rating of 2203.
Lastly, all new products will be ddr4.
Prices of ddr4 are not very different from ddr3(and performance is not so different either)