How can I get the D3.js axis ticks and positions as an array?

Yes, xScale.ticks(4) should give you the actual tick points as values, and you can pipe those back through your xScale to the the X position. You can also just pull the tick points back from the generated elements after you apply the axis to an actual element:

var svg = d3.select("svg");

var scale = d3.scale.linear()
    .range([20, 280])
    .domain([0, 100])

var axis = d3.svg.axis().scale(scale).orient("bottom").ticks(9);
// grab the "scale" used by the axis, call .ticks()
// passing the value we have for .ticks()
console.log("all the points", axis.scale().ticks(axis.ticks()[0]));
// note, we actually select 11 points not 9, "closest guess"

// paint the axis and then find its ticks
svg.call(axis).selectAll(".tick").each(function(data) {
  var tick = d3.select(this);
  // pull the transform data out of the tick
  var transform = d3.transform(tick.attr("transform")).translate;

  // passed in "data" is the value of the tick, transform[0] holds the X value
  console.log("each tick", data, transform);
});

jsbin

In d3 v4 I ended up just parsing the rendered x values from the tick nodes

function parseX(transformText) {
    let m = transformText.match(/translate\(([0-9\.]*)/);
    let x = m[1];
    if (x) {
        return parseFloat(x);
    }
}