Most users of #4 tie line want it to be black, since that is less likely to show up on a stage or in a film or video.
This material starts out as cotton thread which is white when it is spun from cotton fiber. In order to make it black, the thread is dyed before it is woven into tie line. This assures that it is the same color all the way through the line. Doing the dyeing under pressure insures that each thread is dyed all the way through to its core.
This dye is inert, meaning that it will not fade or streak and will not show up on your hands or on other set materials.
The dyeing process in modern textile manufacturing is computer controlled to insure color consistency.
Here is a video that shows the process of making and dyeing cotton thread.