Personally I would opt for other, "better" fish with such limited real estate as it is. 4 in a 55 will add considerably to the bioload especially when they get larger and i'm willing to bet that down the road you'll realize you would rather have some cool wrasses or something instead.
It took a long time (about a year) for my 4 chromis to start widdling each other down, but i've officially started losing them one by one. A few months ago, the smallest started hanging out in the corner by himself all day long. Yesterday he died, and now already the smallest left who used to hang with the group is all alone exhibiting the same behavior as the previous. Overall, it will eventually happen in my experience and there are so many other better choices. But hey, its all about what you like, right!