I have had Cochins for, goodness, a decade now I think, at least if you count the little guy that started it all way back in 2013! So I am coming from a place of an abundance of direct experience with the breed, here. I agree with the others that that's a cockerel. I see saddle feathers as well, and at 4 months of age Cochin bantam pullets still look like your Red girl in the face, a hint of color starting but hardly any comb or wattle growth at all. No 4-month-old Cochin bantam is going to lay you eggs; that's only about 16 weeks of age for a breed that often doesn't come into lay until closer to 30 weeks of age, sometimes even later than that. So the eggs are coming from someone else for sure. I also suspect that he's younger than you were told, or perhaps he's the 3 month old and the Red pullet is 4 months old?
As others have said, mounting behavior does not guarantee that the bird being mounted is a female; often it's a display of dominance and happens with either sex in either position. Very young cockerels are frequently the target for this from older males as they're too young to fight back and very submissive still. I have 3-4 month old cockerels who are still being chased and flogged by my hens, not even the adult roosters in the flock.
I have 3 little girls just like yours but mine are a different color. I do think the black one is a pullet. All 3 of mine have similar looking combs and wattles and lay the exact same size and color of your eggs.
, as is males mounting males. In fact, even my young gamefowl have done it, before the age where they wanted to kill eachother
) soda can I took out there with me just now, but their size compared to it was pretty spot on to the size of your bird next to your soda can. It makes a lot more sense that this is a younger cockerel than this being a bizarrely tiny but much older pullet who has masculine saddle feathers emerging and huge feet like a cockerel would, but also is somehow laying eggs. 
