So ignore the ones that don't workout at all. Now probably ignore the ones just going through the motions at the gym and clearly aren't trying to achieve real muscle growth.
It stands to reason that if you see someone fit and with decent muscles, they're part of the group that might have taken them.
If it's 4% of all males, makes me think it's close to 25% of males with good amounts of muscles. Of course it's skewed by basically 90% of people with massive amounts of muscles taking them. But massive muscles alone doesn't make up 4% of the population.
Even if you set survey methods aside, it has been well established that there are is a single digit percent of people that are simply survey obstructionists. They will say that the sky is red on a survey, because they dont care or are trolling
Reasonable guess, but hard to verify.
It appears for example that every youtuber who looks like they have decent amount of muscles are going to be hit with the accusation that they used steroids.
My perception of most gym goers are ordinary folks who want to get healthy, not gymbros who want to build giant muscles. Of course, this is at a single gym so this is anecdotal.