P4M-328 was a planet designated by the Tau'ri in the Milky Way, home to a feuding, technologically sophisticated people. The planet was believed to be the location of the Key to the Clava Thessara Infinitas.
The Dial Home Device for this planet was lost long ago. Archaeologists found fragments of it, and diagrams were used to create a copy on display in Cicero's Museum. Oddly enough, the fake DHD displayed the point of origin for Earth's Alpha Gate instead of P4M-328's true point of origin (which would have been on the original DHD the replica was based on). The planet's Stargate was a museum piece there as well.
Exhibits in Cicero's Museum included replicas of Ra's headdress, the helmet of a Horus Guard and the uniform of a Serpent Guard. This would seem to indicate that P4M-328 was under the control of both Ra and Apophis at different times in its history.
In the aftermath of the crisis caused by members of the planet believing SG-1 to be hostage takers, it's presumed that the Stargate was either buried or permanently shut down, causing Stargate Command to believe that the planet itself had no interest in establishing relations with any other planets whatsoever. (SG1: "Bad Guys")