OP, I don't know your situation, but I'd kindly suggest that if there are financial limitations before get get a dog, then you're not in a position to get a dog. Large dogs come with large expenses, and you don't want to be in a situation of not being able to afford an emergency vet bill.
As it stands, a 4-ft chain link fence will be okay with 100% supervision. I wouldn't leave a dog there alone for even a minute. Being able to see through the fence will give your dog an opportunity to react to strangers outside, and will give foolish strangers an opportunity to reach over and attempt to pet the dog. Both scenarios can be a problem.
A solid, 6+ ft fence is the only way to go if the dog will be unsupervised for any length of time.