There’s 2 support articles describing the requirements for CPU / (i)GPU with regards to regular and hardware accelerated transcoding.
That being said, there’s no black & white answer to your question. This will highly depend what source media you’re trying to transcode to what output. Also… not all transcoding can be handled by the (i)GPU. On the other hand… where hardware accelerated transcoding is working, even the iGPU of an i7 (Kaby Lake or newer) can handle that.
If you expect your server to do the transcoding by the CPU (e.g. if you have image based subtitles and your client apps cannot handle those), you should look for the PassMark score (cpu score) of the processor as an indicator/benchmark. Transcoding 4K media requires a PassMark score of approx. 12.000 (SDR) or 17.000 (HDR).
For reference… some details about the CPUs you have mentioned in your post:
| … | i5 4670K | i7 7700K |
|---|---|---|
| PassMark score | 7492 | 12089 |
| Can transcode 4K (CPU) | no | at best from SDR |
| HW Transcoding | only for h264 | h264, h265/HEVC (8/10bit → SDR/HDR) and others |
https://support.plex.tv/articles/201774043-what-kind-of-cpu-do-i-need-for-my-server/
https://support.plex.tv/articles/115002178853-using-hardware-accelerated-streaming/
