I'm quite sure that this will turn out to be a cleanout
for the main drain line connecting to the MUNICIPAL SEWER
When the coffee water passed by and you could see it I was sure this was a cleanout. It goes out front... a strong indicator that it is headed toward municipal sewer. Spetic systems primarily go out the back of building...
To confirm this you should just need to call your local city/town hall
and tell them what you are trying to do. They will have a number where they keep the records on this kind of stuff...
The water department probably can't help much here... besides giving you the number to the records dept...
You may need to go to the deeds/records office
and pull up all info. On your property to find the details... but it should be there.
For now, I would leave the drain capped as you have it, but once all is confirmed you will want to purchase a 4" cleanout and cap and cement it into place.
You would purchase 1-4" female adapter, 1-4" cap, small can of PVC cement, small can of PVC primer, and a small can PIPEDOPE (to seal threads on cap).
Let us know what you discover... OK?
PS... shame on the home inspector