I found some sources of definite information, so I will answer this to confirm the suggestion made in a comment that Federico referred to in his answer. Also to fill in the details of what the OP asks.
Triangles 4 and 5 indicate the altitude difference between ISA pressure at sea level and the pressure set in the Kollsman window.
TM 1-205 Air Navigation 1940, Chapter 1, Section VII, Paragraph 52, Altimeter, beginning on page 66, at https://archive.org/details/TM1-205 has an explanation about these triangles, or as they were called "reference markers".
This post at https://www.pilotsofamerica.com/community/threads/blue-and-yellow-does-anyone-remember.79471/ beginning at post #11 discusses altimeters with these markers. Post #25 has a link to an article in the January 1948 issue of Flying Magazine that has a very good explanation of altimeters and these reference markers.
Triangle 5 shows if an altitude difference is higher or lower by which side of the 0 it is on. A higher difference is to the right of the 0 and a lower difference is to the left of the 0. This is in thousands of feet.
Triangle 4 reads in hundreds of feet. Which way you read it depends on which side of the 0 triangle 5 is on.
In the image in this thread, the altimeter setting is 30.1. To see what the difference should be, I used TM 55-6610-247-40 Table 3-1. Altitude Pressure Table, Feet VS Inches of Mercury. I got this TM from http://www.liberatedmanuals.com it says that the altitude difference would be -165 feet.
Triangle 5 is to the left of the 0, between it and -200 feet on the thousands scale. Since triangle 5 is on the left side of the 0, triangle 4 is read from that side and shows -160 feet.
A couple more examples from the thread referenced in the OP What type of altimeter is this?
In the top image the Kollsman window is blanked out. According to TM 1-205 page 68, (3), this is because the limit of the pressure range has been reached. In this case, at the upper limit. The triangles indicate approximately -2380 feet, which would be in excess of 32 inHg.
The bottom altimeter is set to 29.16. According to the table mentioned above, the difference is 711 feet. The triangles are showing this value. Triangle 4 is read from the right side of the zero, because triangle 5 is to the right of the 0.