I think they are all nice names. Are you considering using the name Jacob/Jakob (although spelt with a k, it is more often pronounced "Yakob" and shortening it to Jake as a nn?

By the way, I dont think Aidan is all that common, I only know one boy called Aiden and he is probably in high school now. I work in childcare, so I see a lot of names. Of all those names, Jake is probably the one I hear most.

I'd use Ronan. It is different, but at the same time it fits very well with your girls names. Aidan (I prefer this spelling) is too popular and so is Jake.

I love Jake. its one of my favorite names and had we realized when we named our fist son James that the names were related and meant the same thing we probably would have used Jake instead but we didnt and we wanted to each use one of our grandfathers names. The others... well... Aiden is too popular. Lots of kids r named aiden now or something with aiden in it. And Ronan, well i just hate it, no offense intended to anyone who is named that or loves the name, its just not my style at all.

I like the look of a nice pattern and i noticed you have Grace (g), Bridget (b) and Gianna (G again). i dont know if you considered using a b again to fill out the pattern. If not heres some suggestions.

Ben
Blake
Brayden
Brian
Brody
Bryce
Bailey
Brandon
Brendon
Bradley
Brady
Brett
Bob
Brennan

My first son is names Aidan James and my second son (born three weeks ago) is Ronan Matthew...people had a really hard time with Aidan when I named him that-they thought it was soo different-and as it turns out there are SOOOOOO many Aidan's out there now.
Re: the name ronan-go for it if you like something a little different-we have had mixed feedback from friends about this name-not everyone loves it-we do-but be prepared for people to either love it or hat it-it is definitrely a strong name!(:
If I had had a girl I was definitely going to use the name Grace.
the name jake has never appealed to me-I always thinks of people saying "jake the snake'..wasn't he a wrestler...but having said that-it is a more common name that everyone loves. It has been popular for years so I guess you cant go wrong....

Jake Ronan is a great name, if that is what you have decided upon.

But if you are still undecided:

Grace, Bridget, Gianna, and Jake (I like it, I know one Jake)
Grace, Bridget, Gianna, and Aiden (I like it, I don't know any Aiden's personally but I've seen a couple on TV)
Grace, Bridget, Gianna, and Ronan (I think this one is my favorite, unique)

Good Luck.