generatorsRequired(player)

function generatorsRequired(player) {
  if (player == null) {
    player == me;
  } else if (player<0 || player>maxPlayers) {
    return 0;
  }
  var num_res  = enumStruct(player,RESOURCE_EXTRACTOR).length;
  var num_gens = enumStruct(player,POWER_GEN).length;
  var gens_needed = Math.ceil(num_res/4);
  return Math.max(0,gens_needed-num_gens);
}

// build generator if needed
if (generatorsRequired()) {
  // tell a free truck to build a power generator
}
 
// build all required generators
var i;
if (i=generatorsRequired()) while (-1<--i) {
  // tell a free truck to build a power generator
}
 
// how many more generators does player 3 need?
console("Player 3 needs "+generatorsRequired(3)+" more generators.");

function generatorsRequired() {
  return (enumStruct(me,RESOURCE_EXTRACTOR).length > enumStruct(me,POWER_GEN).length*4);
}
 
// do I need more generators?
if (generatorsRequired()) {
  // build a generator
}