Hi Jade697,
Don't limit yourself to sports related injuries, one of my case studies had knees in permanent flexion, another was in the final stages of healing with a frozen shoulder. Talk to friends/family/colleagues there's bound to be someone with a dodgy knee, aching back etc., apply what you've learned to the injury.
24 case studies was the minimum on my course, it took at least until the third treatment before I felt confident with my assessment techniques, 4 doesn't seem anywhere near enough independent practice. If time allows do extra treatments, I'm sure you won't lose marks for it.
Good Luck.
Wesam