They are sooo young that it seems that the chances are very high that they'd all get along just fine if the introduction was done gradually. Some cats don't react well to being taken out of their environment and he was probably really intimidated by 3 other cats coming at him at once. I'd hide in the corner too. I know they are littermates, but they probably don't realize that at this point and should be introduced again.
There are so many threads on this here! Just do a search to find one about introducing a new cat into the home with a resident cat.
yeah it just sounds like that fourth one needs more time... you can try introducing them by keeping him separated for a bit and having them sniff eachother under the door, swapping blankets so both parties can get familiar with eachother's smells... and if that doesn't work, a gradual approach using vanilla on the back of each cat's necks so they all have the same scent.
there are a lot more details on this forum if you do a search, but i think the above is the general gist of it. i introduced my pair of kitties by swapping them in and out of my bathroom and swapping blankets... partially b/c i had to keep Tyra secluded since she had just been spayed and she needed to heal. after a week, they were both extremely curious about eachother, and now they're best friends!
When I brought my two kittens home one of my roleplay friends said her vet told her to put butter on all her cats when they brought a new one home. They'd lick each other clean with the butter, grooming, not because they trust the other cat, but because they like the butter, and it'd jumpstart the bonding process. (And it worked in her case)
Keeping the introductions between the cats slow might also help. Spaying/neutering/etc may be too soon at this point but it will also, supposedly, help certain behaviors.