Okay, this may sound strange, but just consider it...
I was in the same situation about a year ago. I had a cat who had just given birth to 3 kittens about a month before when I found this poor kitten near the expressway. When I brought the new kitten in, I thought "Perfect, he can just feed off the momma cat and no problem, right?" Wrong. A big wrong. The momma cat was behaving exactly the way yours was. She was hissing and even trying to hit the poor new cat. I tried to sneak the kitten in a few times but every time, the momma cat would see even just a hint of orange (the color of the new cat) and she'd go nuts. Yeah, I know cats are color blind, but I swear, this one new orange when she saw it!
Anyhoo...I was about to give up when I had an idea. I knew that I usually could get away with sneaking the new kitten in with the three others for about 10-20 seconds, while keeping the momma distracted with a toy or a jingling noise (nothing loud enough to startle her though, because then she would get up). So, I sneaked the kitty #4 in, while distracting the mother, and then placed a TOWEL over the kittens, successfully hiding them all from the momma's sight!
Now, I thought this surely wouldn't work, but it did. The kittens didn't seem to mind having a towel drapped over them (I sat there for the most time, holding the thing up a bit to let them breath) and the momma didn't notice the new kitten. I did this and then in between (in small intervals of time), I would allow the momma cat to smell the kitten (if the kitten was in my hand, she would hiss but not attack. I think a cat knows your hand and won't attack; but if a kitten is alone, well that is a different story). Eventually the momma cat got used to the kitten, but it took a while, about 2 weeks. By the end, all four cats were happily eating from the mom, together and towel free!