I'm quite certain that rice is 3C. Adding 4C might be as useful as putting fish gills in a giraffe.
So many other critical things need to be changed as well.
Plants take in carbon dioxide and a major result of photosynthesis and metabolism is biomass = more plant. Apples, whatever.
OK. Answer this question: when a plant takes in a carbon dioxide, where do you first find that carbon atom from that carbon dioxide?
Answer: The carbon atom is combined with a 5-carbon molecule and immediately split in half = one of the 3-carbon bits has the new carbon in it.
That is the "opening step" in the Calvin Benson cycle in chloroplasts.
However.
There were a couple of guys working for Dole Pineapple in Hawaii who notices that pineapple plants didn't perform as expected.
They got paranoid about losing their jobs so they passed their results to some other guys at an Australian university (Brisbane?)
Hatch and Slack soon discovered that elsewhere in the leaves of many plants, the carbon dioxide was being almost vacuumed out of the air.
The detail is that the new carbon shows up as a part of a 4-carbon molecule in very specialized chloroplasts in very specialized plant cells.
It's a really efficient uptake, far more than the CB cycle alone. This is why it's so important. Called the Hatch-Slack Pathway.
Next,
These plants split the 4C, and recycle the 3C part while the new carbon gets handed off to the CB cycle, as usual.
These days, many different plants have been found which exhibit 4C.
Most of the time, the plant anatomy, the arrangement of the living plant cells in the leaves, is enough to give it away.
Ecologically this does not mean that these plants require an equivalent uptake of everything else. More, yes. Equal, no.
Humans have a really bad habit of removing the most nutritiously valuable plant parts
and rarely if ever do people put any of those nutrients back where they came from.
Yeah, I'm talking human waste. Western social stigmas prevent that. Fix it.
In asia, they know better. Called "night soil."