Doisac is a very war-torn land due to constant Jiralhanae civil warfare, typically between clans. In their wars amongst clans, they used nuclear weapons, reducing their culture back to its primal state before the discovery of space travel. When discovered by the Covenant, the Brutes were just starting to re-discover radio and spaceflight. During the final months of the Human-Covenant war, it is more than likely that Doisac was the site of several battles between the Covenant Loyalists and Separatists.
The statement that Doisac is a "Native Tier 4" refers to the Technological Achievement Tiers. The Jiralhanae had worked their way up to this level before a devastating civil war brought the Tier level down again; eventually they were integrated into the Covenant with Tier 2 (Interstellar) technology level.
From the limited details available, it can be estimated that Doisac's environment is very Earth-like, with forests and oceans. It has been noted, however, that its terrain is dominated by magma and magnetism, hinting at high tectonic activity and rich magnetized iron deposits likely caused by the lunar tidal effects projected by its three moons. It is possible that Doisac is closer to Oth Sonin than Earth is to Sol because its surface temperature/climate is slightly more tropical, ranging from -15 °C to 52 °C (5 °F to 125.6 °F). Alternatively, its atmosphere may cause a natural greenhouse effect, trapping heat and warming up the planet in a similar fashion to Venus.