ok well first of all i'd rec everything hotarumyst has written for b1a4. i thoroughly enjoy everything she's written in ficdom but i can feel that she's very very attached to b1a4 from the way she writes about them. i'm not too sure what you'd like, but two i can rec are:

http://ourhandsfroze.livejournal.com/24882.html - au. b1a4 are a group of cosplayers (geeky fun and costumes and porn)
http://ourhandsfroze.livejournal.com/24574.html#cutid1 - b1a4 rockband!au. cnu/sandeul and sandeul/jinyoung friendship.

2nd fic is based off bulletthestars rockband drabbles: http://bulletthestars.livejournal.com/12688.html#cutid3

another favourite of mine is this canon!verse cnu/gongchan fic http://mammothcock.livejournal.com/27015.html

you could also check out the b1a4 secret santa comm to see if anything is to your tastes too! http://b1a4ss.livejournal.com/