With Cochins (especially bantams) you will always know for certain by 8 weeks. Most of the time you know by 4-5 weeks. If a pullet had that much comb and color (especially wattles), she's at POL.
Like you've read, Cochin cockerels color up and show their gender very early. On the complete...
4 weeks is a bit early, but Cochin bantam cockerels generally like to show their colors early. Your first buff is absolutely a cockerel. The blue looks questionable and so does the second silver penciled. All the rest appear to be pullets right now. Post updated pictures in 2-3 weeks and we...