Do we skip crossfire events in Extraction when playing with less than 4 players?
Yes, you will still flip crossfire cards as the rules for extraction say to follow the crossfire rules of a standard crossfire game. In a standard game you flip it on the first turn with four players, second turn with three players and the third turn with two players. The only change to what is being flipped is obstacles which are flipped according to a tracker in the mission instructions.

Need to remember that crossfire cards are where a lot of the difficulty comes from. I have lost count of how many games have been lost in the last round just because of the crossfire card that is flipped.

Rules

Page 20 of the rules


  Obstacles and Crossfire: Use the normal Crossfire rules. Flip
  obstacles as instructed for each step of the mission track.
  Flipped obstacles start facing the Client, but the runners
  can protect the Client. At the start of each runner’s turn, a
  non-staggered runner can move one obstacle facing the Client
  to face them instead, if that obstacle’s color matches their
  role color. This choice must be made before any obstacle abilities
  that would trigger at the start of a runner’s turn.
  Obstacles will attack runners they’re facing, as normal, but
  when a runner becomes staggered, move all obstacles facing
  that runner to face the Client at the end of that turn.


Page 19


  Playing with Fewer than Four Runners: If there are only three
  runners, do not flip a Crossfire card at the start of the first
  round. If there are only two runners, do not flip a Crossfire
  card at the start of the first or second rounds.