My character has a -1D injury. Her perception exponent is 4B. If she were to roll the test, she’d roll 3B. When she helps another character with an Ob 4 Perception test, do I record the successful test as a Challenging or Difficult test?
You’d log a Test as if you’d rolled 3D vs. Ob 4.
Indeed, it’s a Challenging test.
Helping others is the BEST way to get Challenging tests while having a real possibility at succeeding. Helping others while injured is a GOLD MINE for advancing! 
In fact, one of the easiest way to log Difficult and Challenging tests for high exponents is to arrange to be hindered on the test–whether that’s by injuries, circumstances, or other events.
BW makes healing times long enough that often you have substantial incentive to keep going despite the injury, and the advancement system means that you often will come back stronger than before from logging all those tougher tests!
(So make sure the villain’s really dead, not just injured, or he’ll make you pay.)