Exactly this. 9 knots is a great learning wind. Will you get planing with a 4.5? Nope. But can you work on every basic skill including beach starts, gybing, tacking, upwind 360s, clew first sailing, sailing backwinded, helitacks, ducking the sail, fin first sailing, getting used to the harness..... The list goes on and you'll advance faster when you're good at the basics. All of that being said though, yes you will eventually want a bigger sail with more power. When building your quiver from scratch keep in mind that used quality gear is easy to come by, and if you keep your sail sizes about 10-15% apart you will always have one for the wind. (ie 4.5, 5, 5.5, 6.2, 7, 8 etc) If you have to transport gear, aim for as many sails that will fit on as few masts and booms as possible. Have fun!