Apelles

from Pliny, Historia Naturalis, book 35
(an older English translation is online on the Perseus website)

(55) ways of recording dates

Ramirez read the row of circles at the bottom of the drawing as a way of marking the year 1537, i.e. in the fourth year before 1540. But an easier way to make sense of the date seems to be to relate it to Bartholome de las Casas, even though his Brevísima relación de la destrucción de las Indias was not published until 1552.

The Indies were discovered in the year one thousand four hundred and ninety-two. In the following year a great many Spaniards went there with the intention of settling the land. Thus, forty-nine years have passed since the first settlers penetrated the land, the first so claimed being the large and most happy isle called Hispaniola, which is six hundred leagues in circumference. Around it in all directions are many other islands, some very big, others very small, and all of them were, as we saw with our own eyes, densely populated with native peoples called Indians. This large island was perhaps the most densely populated place in the world. There must be close to two hundred leagues of land on this island, and the seacoast has been explored for more than ten thousand leagues, and each day more of it is being explored. And all the land so far discovered is a beehive of people; it is as though God had crowded into these lands the great majority of mankind.

It takes some mental arithmetic to realize that the book was written in 1542 and published ten years later, and since Las Casas was a priest it seems to make sense to understand the 49 years as an allusion to Biblical time, specifically the Jubilee of Leviticus 25.
If the artist who made the drawing of the events in Coyoacan was also counting forward from 1492 the "four years before forty years" implied by the row of circles with a cross marking a point four units before the end of the row might imply something similar to the way Las Casas reckoned the date of the Brevissima Relacion. It is almost as long as the 40 years the Children of Israel spent wandering in the desert. The "four years before forty years" may be a combination of a Biblical unit of time and an American preference for exact dates. Counting forward from 1492 and adding 36 completed years would be equivalent to 1528, before Cortés returned to Spain.
Even though the drawing obviously represents specific events, and whether or not the 40 circles have to do with counting forward from 1492, they seem to be a way of emphasizing, as Las Casas did, that similar incidents took place over a very long period of time. There is something similar in The Garden of Delights/El Jardín de las Delicias where an incident of a person being attacked by dogs is located near a date sign that in the context of a series of dates make the most sense as 4-Rabbit, then again as 8-Rabbit. (Looking at photographs taken before the triptych was restored, the sides of the cube were apparently once two 4's and a 2, as though someone were cheating at a dice game.) Similar incidents might just as well have taken place in 2, 4, 6, 8, or 10-Rabbit (or any year, taking into account all six surfaces) and the seeming imprecision emphasizes that as Las Casas kept saying, such incidents were ongoing.