That isn't what is happening here. Colorado's border was defined by congress as 37°N to 41°N latitude, and from 25°W to 32°W longitude. That is effectively a trapezoid or at least as close to one that can be drawn on a globe. That shape does have 4 edges as you say. However, when they went to survey the land errors were made. 19th-century surveyors would stray from the legal border by as much as half a mile in some places and would correct it by making a couple of near 90-degree turns to correct this. Congress then accepted the survey boundaries as the legal boundaries making it have more than 4 edges.