Well... as weird as Fifth Harmony sounds with four members, it might've been a bit weirder to change it altogether.
"We felt like within ourselves, no, Fifth Harmony is the name we grew up with and we worked for and it's our brand. It's who we are. It's our entire moments. It's what we spent five years on."
Here here girl. Not to mention the most important reason:
"The fans names us Fifth Harmony," Dinah Jane Hansen explained. "And now they're the fifth member."