Do you think 4 cats is too many?

I live in a two bedroom apartment. We have ample space in the apartment and we currently have 3 cats. When you walk into our apartment it is very clean and smells nothing like cats. It generally smells like new apartment actually. Anyways, two of the cats are mine and one is my boyfriends. We've been living together for 3 years now and I had one cat, he had one cat, and we got the thrid cat back in 2008. We also have a toddler and he loves them and they love him so there is no problem that way. Anyways, my boyfriend's mother was looking for a friendly cat as well. She had gotten a cat last year but this cat is not nice to anyone except her. To make a long story short I found her the friendliest cat online. He is only a few years old, but he loves to cuddle and be held. He is my perfect cat cause none of my cats like to be held like this cat. Anyways, she's decided that since this cat and her mean cat don't get along that she will get rid of the cat I helped her find only two months ago to keep the mean cat. I felt so guilty that she was getting rid of this nice cat and I wanted him from the moment I found him for her. I really want to keep this cat but my boyfriend is freaking out saying that it will be too many cats and to difficult for the other cats. I feel like as long as things are kept up and the cat is well taken care of what's the harm if we had just one more? I am not an animal hoarder by any means, but I feel like I won't find another cat like this one (extremely nice, raised around people since birth) ever again. I don't want to go against my boyfriend's wishes, but I'm the one that will be doing all the work including financially. Is it wrong to get another cat?