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"Command HQ, this is Fireteam 4. We are pinned down in Alpha Labs."
― Fireteam 4, reporting at the outbreak of the demon invasion  [src]

Fireteam 4 was a team of Marines stationed on Mars City in Doom 3.

When the demons invaded Mars, Fireteam 4 ended up pinned down in Alpha Labs and later reported they were attempting to secure the area. It is highly unlikely they survived, given that Alpha Labs was overrun and they were not heard from after their report to Command HQ.