I moved this to the Hardware Forum where you are more likely to get a technical response.
That's a tough question and I'll start with "it depends".
What is your workflow and media?
Do you use any 3rd party add-ins that only support single-threaded operation?
What are the specs. of your whole current system?
Mainly I'm rendering clips and films using built-in filters and effects (color correction, stabilization and so on). I want to speed up the real-time performance in Premiere with 4k (and higher resolution) video in h264 codec and in RAW as well and also reduce rendering time of long films (2 hours and more).
These are the whole specs: HP Workstation Z840 wih Xeon 2620V4/16Gb DDR4/Nvidia Quadro M4000 8GB GDDR5/500Gb SSD (for system and apps) + 8Tb SATA HDD (for video)/Win10.
So is it better to replace this CPU with for example 2650v4 (16-cores) or I should only buy the second 2620v4 (8 cores) to get the same performance boost? Does Premiere can properly work in doulbe-cpu system?
Or may be I should think about second GPU?
Adding a 2nd 2620v4 would be WAY better than replacing the current single CPU with a single 2650v4.
Premiere's support of dual GPUs is marginal. They can assist with rendering, but only a single card is in play for real time playback.
I'd suggest adding the CPU and possibly 16GB more RAM considering what you have now and your workflow.
Jim is right that your 2 x 8 cores is better than 1 x 12 cores and the speed is very close to the same. If you want to see some dual Xeon Premiere Pro CPU intensive scores see my PPBM CPU data
I do not have any v4 Xeon scores but you could test before you upgrade and Submit the results and I would be delighted and comment on your scores.