The MSS collects four bands in total. Band 1 includes energy in the green band, band 2 covers the red, and band 3 senses a different part of the infrared than band 4. Although the image is displayed in black and white, you should think of the display as relative strengths of infrared, with lighter areas stronger in infrared. The image probably looks somewhat dark. The satellite image only sees what is reflected by surfaces on Earth.
Most objects tend to absorb infrared rather than reflect it.
Vegetation, though, tends to reflect infrared energy, so the brighter areas are probably high in plant life. When Brazil4 is on the screen, click the cursor inquiry tool on as you did earlier to examine data values. Move the cursor to some part of the image, and press the left mouse button. The z, or data value , of the cell should be shown in the status bar. Now move to another few spots and repeat the procedure to get an idea of the data values in the image.