3N and 4N give very different looks. I am NC30 and I ended up purchasing all of the l/s and l/g from the N Collection because all of them worked on NC30 skintone!
3N is great for a nice sexy light pink lip. Similar to many MAC pink shades, but like all good LEs, is a stand-alone color IMO. It's the perfect lipstick for a cool look--pink/purple/blue eyeshadows would go well with it.
4N is warmer. Think of paring it with Spice or Cremola liners. It would go well with warmer eyeshadows--bronzes, browns, tans, golds.
So if you can get the actual colors, it is WORTH the money. Both are non-regrettable buys because they aren't 'trendy' colors.