The passage's core claim is that Hatha Yoga's methods are so hard that only a few resolute people manage to complete all its stages, some have even failed or died trying, while Raja Yoga is not described this way. To strengthen this claim, we need evidence that actually measures difficulty or success, not just how many people are involved or how many schools exist.
Only option 4 measures actual success rather than popularity or availability, so it gives real support to the passage's idea about how difficult and risky Hatha Yoga is. Option 4 is correct.
Let's summarize:
The passage's idea is strengthened by evidence that a smaller percentage succeed at Hatha Yoga than at Raja Yoga, which is option 4.