Catan settlements on roads
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.