When discussing ideas for Mass Effect 4, many fans bring up the First Contact War as a good time period for the game. ME4 needs a better idea than this, though.
The First Contact War, if you're not aware, was a conflict between the humans and turians. The two races fought each other for three months before the Citadel Council intervened. The war was so named because it was the first time humans had encountered the turians, or any other alien species, in their history.
Without knowing anything else about the conflict, you might think this would be a great premise for a prequel. Players get to see younger versions of many pivotal characters from the trilogy like David Anderson and Steven Hackett. Plus, it's the only major war that humanity took part in before the events of Mass Effect so it seems like the default choice for a prequel Mass Effect 4 if you want humans to be involved.
However, the First Contact War wasn't as epic as you might think. It was a pointless and avoidable war started by accident.
The root cause of the war was the mass relay network. These relays, relics of a dead species, allow for speedy travel across the galaxy. After finding a mass relay in Pluto's moon, humanity began to expand beyond their solar system. They activated every relay they could find, using them to move farther and farther across the Milky Way.
Humanity was unaware, however, that reactivating inert mass relays was forbidden by the Citadel Council, the ruling body for much of the galaxy. This law was put in place after salarian explorers opened a relay to an uncharted world and unleashed the rachni, a violent race of insects that then tried to take over the galaxy.
The human explorers trying to activate Relay 314 didn't know about the Citadel or their regulations. The turian force that caught them, however, did. Instead of introducing themselves to this never-before-seen race, the turians opened fire.
After a few small battles between the two races' fleets, the turians began an assault on the colony world of Shanxi. They destroyed the ships in orbit and then began to pummel the human forces from orbit. General Williams, grandfather of Ashley Williams, was forced to surrender the colony. The turians couldn't revel in their victory long, though, because Admiral Kastanie Drescher and the Second Fleet soon showed up to drive them from the planet.
Before the war could escalate any further, the Citadel Council stepped in to stop the hostilities. The turians and humans each suffered about 600 casualties.
In summary, it was a dumb war. Two races stumbled into each other and started swinging. There was no reason for the war and little was won or lost.
The First Contact War isn't an uninteresting conflict. It had some important diplomatic ramifications. The war is why other races treated humanity with such distrust at first. It's also why Shepard's appointment as Spectre, and the collaboration between humans and turians in building the SSV Normandy, were such momentous occasions. The conflict also explains the rise of pro-humanity terrorist group Cerberus.
It is, however, a small conflict. Mass Effect 3 depicted a galaxy-wide war with a death toll of millions. Would BioWare really follow up that game with a war with a mere 1,200 casualties, most of which were from orbital bombardment or space battles? It's unlikely that the next Mass Effect will be able to depict a war any larger than the fight against the Reapers but the First Contact War is too much of a downgrade.
There are too many better ideas for Mass Effect 4 out there. The First Contact War inspired a comic book and doesn't deserve much more than that. BioWare has more interesting locations and events to explore with their game.