I tend to prefer to play four women when it's a windy enough game that everyone is struggling to score upwind. If the wind is such a factor that its tough to score upwind, you really want to limit the other teams ability to score upwind since an upwind score is so critical to the outcome of the game.
With offense continually going downwind, they get to decide the make up. Choose four women and three men so that the other team has to work it upwind with one less male. In general, men tend to be "stronger" throwers and work it better in heavy wind. So, to limit the other team and its upwind scoring chances that much more, take one of their men off the field.
Beyond that...play four players when you have four players that are much better then the other teams four players. Use the ability to change the gender mix to focus the play towards your teams strengths, and highlight the other teams weaknesses. If your opponent has great men, but poor women...play four women when possible. If your opponent has great women but poor men, play four men.
Michael Faris was a founding member of Trigger Hippy, and is an influential player and deft handler living in Montana. Michael played in 2007 with the Flycoons, and continues his success with Big Sky Mixed Ultimate.