How do I add a traffic signal with the online editor (there are four traffic lights at a crossroad)
add one or more nodes following the instructions described in the OSM wiki: https://wiki.openstreetmap.org/wiki/Tag:highway%3Dtraffic_signals
Normally people initially map the traffic signal on the single node of the intersection. This is fine for most routers to add appropriate penalties, and perhaps indicate that a driver is approaching the traffic signal.
Once one wants to include things like pedestrian crossings, advanced stop lines for cyclists etc, then it is best to map each individual traffic signal on the relevant way (usually the visible stop line is the best place to locate it). You can also add a direction tag (forward/backward relative to the way) to indicate which flow of traffic is affected, but I dont think this particularly common.