Scans showed their brains underwent three years' worth of development in just three months.
At the same time, they displayed remarkable improvements in tests of reading, concentration, problem-solving and memory.
One boy who previously scorned books and was hooked on TV developed a love of reading and declared he was "bored" of television.
The three boys and one girl, aged between eight and 13, were taking part in a pilot study looking at the effects of diet on developing young brains.
Scientists believe the results are powerful evidence of the harm "junk food" is doing to Britain's children.
All the children were classified as overweight. Zach, aged eight, weighed eight stone, George and Rachael, both 11, each weighed 11 stone, and 13-year-old Gareth weighed 12 stone.
At the start of the study the children were given a supplement called VegEPA, which contains a combination of omega-3 and omega-6 essential fatty acids.
They took two capsules a day for three months and were also encouraged to cut down on fatty snacks and fizzy drinks and be more active.
At the end of the study period the children were put through a battery of tests and given hi-tech proton spectroscopy scans that look at biochemical changes in the brain.
Dramatic improvements were seen in every area. The children showed an increase in reading age of well over a year, their handwriting became neater and more accurate, and they paid more attention in class.
In one measurement of concentration, called the Stroop test, three children scored perfect results.
But the most surprising findings emerged from the brain scans.
The scanner at St Georges Hospital, London, resembled a conventional MRI (magnetic resonance imaging) machine. But it was more sophisticated, and able to detect a key biochemical indicator of brain development called NAA (N-Acetylaspartate).
Higher levels of NAA correspond to more nerve fibres growing in the brain.
Professor Basant Puri, from Imperial College London, who led the study, said, "The results were astonishing. In three months, you might expect to see a small NAA increase. But we saw as much growth as you would normally see in three years.
"It was as if these were the brains of children three years older. It means you have more connections and greater density of nerve cells, in the same way that a tree grows more branches.
"For all the children, there was a marked change, but in the three boys, there was a massive, massive increase in NAA. I was quite startled by what I saw."
The growth appeared to be spread throughout the cerebral cortex, the "thinking" part of the brain.
Prof Puri said the parents of the eldest child, Gareth, were astounded by the change in their son.
"Gareth's parents told me how he had suddenly found TV boring, as he wanted to read," he said. "Three months earlier he was saying he couldn't understand people who loved books.
"Once they have this better attentional ability, children have a much easier time producing a continual narrative in their heads as they read, so reading becomes more enjoyable.
"The concentration of all the children improved enormously and they seemed a lot calmer and happier. Even before I started testing them, their parents were saying how much better they were."
Although the children were asked to change their diet, there was no evidence they did to any great extent, said Prof Puri.
He believes the effects were exclusively due to the supplement, which is derived from oily fish and an especially pure form of evening primrose oil.
It contains an essential fatty acid called EPA, but significantly, another type of fatty acid, DHA, is absent.
Previous studies by Prof Puri have shown this formulation can improve brain function in adults.
Imaging expert Prof Kishore Bhakoo, from the Medical Research Council Clinical Science Centre at Imperial College, who was shown the scans, said, "The thing that amazed me was how much change in biochemistry you could see in three months. I've never seen anything like it. You'd expect some variation, but they were all going in the same direction."
He said the results had important implications for the "junk food" debate.
"Processed food doesn't contain these substances," said Prof Bhakoo. "A chicken from a factory farm is white, but the proper colour for a chicken is yellow."
Prof Puri said he wanted to repeat the experiment with a larger number of children, if the funding was made available.
He added, "I believe this does point to the conclusion many scientists are coming to, that these fatty acids are particularly good for brain growth.
"It does make you think about education and whether we should be giving all children these supplements."
From the March 13, 2007, Prepared Foods e-Flash