Is the calyx, the female plant organ in cannabis, visible without going into 12/12 light? So, does cannabis show gender in the 20/4 light program?

Yes, (feminised) photoperiod plants will show pre-flowers (calyxes) once they are sexually mature and before actual flowering commences, even when the light cycle is still in the vegetative stage, even at 20/4. It is an indication that your plant has hit "puberty", and not dependant on being at 12/12. Completely normal!

Weed often displays sex before flowering is induced. You'll see them typically where branches and the main stalk intersect. In other words, at the node a female will show a single Calyx with 2 pistils, that's your female. Males will have little dangling sacks. If you have feminized seeds your don't need to concern yourself with early sexing of your plants. They will be all female if they have been feminized.

Well, technically it’s an feminized hybrid. The light cycle trigger only the flowering, the feminized hormones are already in the genetic code of the plant even in Veg period. They start to show during long Veg cycle.

They will shoot a pistil or two at node's as they get into maturity

preflowers can show up before flower phase begins, yes. Typically, with a long vege period you will see them, but no guarantee they show up before flower phase. this is true of male plants, too. It is not restricted to feminized plants. Early on it may be hard to tell the difference, but long before it can produce any pollen, it will be very easy to discern a male and female preflower.

feminized plants do not show sex until your light cycle is shortened to 12 hours of light per day. Usually, within 10 or more days after that, the plant will show sex. Female plants will show a small sac with two pearly white pistils protruding. If you never change to a shortened light cycle, the plant will stay in vegetative growth mode.

That is very much dependent by the strain and genetics , some could show calyxes after 6+ weeks of veg , but I haven't really seen anything before that .