Whether there's an obvious gender difference at any age depends on genetics, environment, diet, and so forth far more than it actually does on breed, regardless of what some of the books will tell you. I read one chicken book recently that horrified me with the near-death state of some of birds they used in the breed example photos, and most of the birds in that book were hideous, unwell and terribly inbred and seriously looked like they'd been buried underground and exhumed for the photoshoot... It was terrible! Multiple serious genetic defects, deformities, the 'expert' writing that book clearly wouldn't know a healthy chicken if you slapped them with it. Sorry, that was totally off on a tangent. But sort of relevant in terms of the faulty examples 'experts' can publish like gospel.
Rearing mine outdoors, on as natural and healthy a diet as possible, over the generations resulted in chicks I could tell the gender of as they were hatching with many, and for those I couldn't tell at hatching, I could tell within the first week. Sunlight has a lot to do with how quickly they develop, in my experience. Those reared indoors are mostly less developed even at weeks and months old. It's not uncommon for some to not show gender until they're at the ages when they should be becoming fertile. I don't believe that's an indication of good health though, just my opinion, but yes, by four weeks you should be able to see gender indications in probably the majority of birds of any breed.
Brahmas, from my limited and second-hand experience with them, seem to usually show gender more in their wattles than their crests at first, and they seem regularly slow developers. But as you say, that one looks like a pullet, and if others have larger crests, chances are they're male.
Best wishes.
