(Disclosure: I learned how to drive in Germany, details may differ in other European countries, apart from the obvious mirroring when driving on the left)
The fallback rule for all situations is "yield to the right". If you're at an intersection with no signs or markings, you get to drive if there's no-one to the right of you.
This creates a deadlock when all roads are unmarked and there's a car on each of them at the exact same time but that almost never happens and can be resolved by slowly inching forward if the person to the right of you doesn't move and then exiting the crossing.
When entering from a road that has a solid line or a yield sign, you always have to yield to the traffic on the road you're merging into. A stop sign means you have to come to a full stop even if the road is empty and slowly approach until you can clearly see there's no traffic and only then merge (after yielding).
Most other rules follow from that.
I'm not sure what you mean by "4-way stop" but if there were a 4-way crossing with stop signs on all sides it would result in a deadlock and be resolved the same way. Except with the explicit restriction that everyone first has to come to a full stop and slowly move forward to determine the traffic situation.