The Guideline
Very roughly speaking, for a single full-transcode of a video, the following PassMark scores are a good guideline for a requirement:
1080p/10Mbps: 2000 PassMark
720p/4Mbps: 1500 PassMark
The CPU Benchmark website is a good resource to see what sort of PassMark score a particular processor received.
Related Page: CPUbenchmark.net
How to Use the Guideline
Basically, if you think you may be needing to handle 4 simultaneous content streams and they might all be 1080p content that requires transcoding, then you would take the base guideline (2000) and multiply it by the number of simultaneous streams (4) to get your rough requirement, which would be a PassMark score of 8000 in this case.
In this example, that means you would want to look at processors with an 8000 score or higher. Also, keep in mind:
If you'll be using your computer for other things beyond simply running Plex Media Server, you'll want to make sure you have some extra power so that you can continue performing other tasks
It's a good idea to remember that your usage today may not be the same as your usage in the future. You never know when you'll get a new device to run Plex, share content with a friend, etc., which could all result in more usage.