Let's say I have 4 connected roads with no settlements or cities in between. Can I later go back and place a settlement in there?
Yes, so long as it's not adjacent to another settlement.
A valid location for a settlement is any spot that touches one of your roads, and is not adjacent to any other settlement. I'll also note that it's legal for someone else to put a settlement in the middle of your road, so long as they also have a road that touches that intersection - this will cut your 4-segment road into two 2-segment roads, which can be a useful strategy for killing someone's longest road.
Yes. From the rules:
Each of your settlements must connect to at least 1 of your own roads.