FWIW, don't let that 4 oz get ahead of you. It can be pretty thick for smaller bags, especially when it comes to turning them. Lighter leather than that is definitely easier to work with on your first attempts. Tandy has 2.5-3 oz veg tanned "kip" sides on sale right now at $49, or roughly half price. They run around 18 square feet, making it a heck of a buy for a big piece of leather.
I just finished making one of TC Albert's North Hampton bags out of 4 oz, and it's way thick for my tastes. There are some spots in that pattern where you have to pierce 5 layers of leather with your awl, and that's no place to be on a first build.
That "too heavy" business is a first impression, but frank. I'm going to make another of some of that kip side just to see what's what, and I think I'll like it a lot better.