The 8400 will be a lot faster/better then the G4560 since it does have 6 cores 6 threads with turbo to 4Ghz.
However the G4560 does only have 2 Cores 4 Threads and a Max Frequenzy of 3,5Ghz.
You can still run the PC without GPU by using the IGPU of the 8400, but gaming experience is not that great. It can run Fornite on low settings and Esport games League of Legends and CS:GO for example with low settings on a pretty decent framerate but nothing outstanding.
SO if you dont mind waiting a few months until you can buy a dedicated GPU then the 8400 would be your best option.