Four days might be a bit too early to "know" anything.
There are quite a few different types of shoulder pain as well as countless things that can cause them.
Do you have any other symptoms like catching or clicking?
What line of work are you in?
From your description, you sound relatively educated and experienced.
Try searching (webMD or even Google) by typing in any and all of your symptoms, being as specific as you possibly can. You might be surprised at the results.
Be prepared to do plenty of reading and more searching.
Oftentimes, different conditions can cause the same symptoms.
You may end up having to research things like T4 syndrome, thoracic outlet, etc.
If things don't improve for you in a few weeks, it may be time to see a physical therapist - preferably one who specializes in sports injuries. Believe me when I say that NOT ALL PT's ARE CREATED EQUAL.
Be careful with that shoulder and good luck!