I'm glad you asked this. I'm in the EXACT same situation (a little eerie). Just like you, I'm 23 and the house I'm moving into only has a 4ft. chain link fence. Beau has never been a jumper, but then again, he's never had the opportunity to jump a fence. I don't have a lot of money, either. The solution I came to is this: the neighbors behind me and on my left side have tall shrubs lining the fence, so I don't think Beau could clear them. The side yard faces the front of the house and the street, so I'm putting up a 6' wooden fence. The gate to the yard faces another street (it's a corner lot), so I'm going to fence in that part of the fence, too. This will only be about 30ft. of fencing, but I have to install a gate, too. I know this probably won't help you, but it's the best I could come up with in my situation. Since your boxer is still young, I would just focus on training. Hopefully, he will learn that jumping the fence is not acceptable behavior and you won't have to worry about it. I would NEVER leave him outside unattended, though. My boys won't be outside without me unless I'm cooking (my kitchen window overlooks the backyard). I'm not going to take the chance of having anything happen to them. Good luck!