Why is it called Signal Four?

The name “SIGNL4” refers to certain ways our brain works.

Our brain is able to accurately determine the number of up to four displayed elements within a very short period of time (“subitizing”).

SIGNL4 is an application for critical situations. Under stress the fast and precise determination of circumstances is essential. Hence, SIGNL4 implements a new type of visual alerting that accommodates our brain. It minimizes the amount of information shown and reduces displayed information to colors, sounds and graphical elements as much as possible.

From Wikipedia:

Subitizing is the rapid, accurate, and confident judgments of numbers performed for small numbers of items. … The accuracy, speed, and confidence with which observers make judgments of the number of items are critically dependent on the number of elements to be enumerated. Judgments made for displays composed of around one to four items are rapid, accurate and confident.”