Glyphs on the side panel mention four witnesses of the rituals related to the installation of Stela A: emblem glyphs for lords from the sites of Copan, Tikal, Calakmul, and Palenque. These lords represented the four most powerful political entities of the Mayan world at that time.
"Typically an Emblem Glyph consists of three elements. The first is the 'water group' affix on the top or left edge of the main sign composed of a row of droplets signifying blood, topped by a small sign, usually a k'an sign, a shell sign [as is the case here], or an upside-down Ahaw, the composite standing for ch'ul, and read as 'divine' or 'holy'.
The second element is a superfixed sign for Ahaw or lord, usually the ah po sign [the top middle and left spheres next to the shell sign], and the third element is a main sign that is currently believed to designate the site or polity (i.e., a toponym)."
John Harris and Stephen Sterns, Understanding Maya Inscriptions: A Hieroglyph Handbook, p. 71.
The emblem glyph for Copan, top right, features a fruit bat head which was read xu and combines with the syllables ku and pi to read Xukpi, the ancient name of Copan. The second row shows the glyph for Tikal (which was known as Mutul in ancient times) on the left and Calakmul (represented by a snake) on the right, while the left side of the bottom glyph, with the bones inscribed on a sphere, represents Bak, 'Bone', the ancient name of Palenque. These glyphs would therefore read "The holy lord of Xukpi, the holy lord of Mutul, the holy lord of [Calakmul], and the holy lord of Bak."