Upgrading desktop PC for Premiere Pro

Hi all

I am doing my research to better understand the hardware components of desktop PCs in order to upgrade mine but am looking for some extra advice. The end goal is to have a machine that can process 4K footage with ease, the short-term goal is upgrading the current PC to allow 1080p footage edits with ease. Ideally replacing one component at a time to assess the performance changes.

Here are my current specs:

I understand that for 4K I will work to replacing practically every component. But what is likely the current bottleneck? Premiere Pro runs OK with 1080p if viewing a couple of short clips but starts lagging soon after that. 

If I was to only replace the CPU for now, should I go for an i7 or i9, and which one? Or AMD? Will that be pointless if I don't also upgrade the GPU? Or would even replacing the ram with 32GB DDR4 boost the current performance?

Or should I just proxy my 1080p to 480p for now? That's all new to me but has been suggested by a couple of people.

Thanks a bunch for any feedback/comments!