Breast collars are a great thing to invest in :) they help with lots of things and I'm sure you'd find a use for it.
I got one a couple years ago thinking I'd start jumping but my horse went lane everytime I tried to jump her.
So this year I got my new horse :) she got started in October and has quite the gaits. I use it everytime I ride now! It helps the saddle stay in place when she is dripping or on trail rides. Posted via Mobile Device
You don't really need a breastcollar unless you find your saddle slipping back. My instructor has found that a lot of her Quarter Horses (and my Quarter Horse) need a breastcollar because of their build. Thank goodness for it because my saddle would slide back a lot after one jump! But if your saddle doesn't slide, don't waste your money on it, especially since you'll just be doing 4H shows.
I use one whenever I jump, not only to help keep the saddle in place but as an "oh sh!t" handle on XC when I need it. Look around on ebay/craigslist, I bought my like-new Ovation on Craigslist for $50.