The rafter ties(on the walls) are required at 4' on center, minimum code. You can put them on every one if you so desire. The collar ties may not be required as they hold the roof rafters/ridge together at the peak from separating in high winds/uplift pressures. The walls will bow when you get any snow load with a 2x4 wall, 13' long, that is guaranteed. Don't forget the H-clips, or H- 2.5, at every rafter, both sides:
H Seismic and Hurricane Ties
Be safe, G