Good luck!
I have two boys and three girls now but I started with the boys - they got along really well until the first two girls came into the picture, then they began fighting for dominance. At that point, I had to separate them and they are no longer compatible.
The two girls were fine with both boys separately until recently, one lady was fed up with mating attempts and bit one of the boy's fingers right off (luckily it was between the two bones in his finger which made for a very easy heal and very pricey vet visit). So now, it's down to the two girls and the other male who have a clean history - I'm wary, naturally.
My newest girl I refuse to introduce to the others, as they all have adjacent cages with one another and I can tell they find her scent somewhat threatening. It's been a few months and I would rather not introduce them at all. She's a very happy chin on her own.
So, in my experience, introducing chins isn't always risky, but compatibility definitely is. You may run the risk of having four separate cages for five chinchillas (and it's 4 and not 5 cages only because the two girls are sisters and won't fight over a thing). For you, if you have the energy and time and capabilities to take care of another, so even if the introduction doesn't go well, you'll be okay with that and carry on with another chin in the family!