Of the 4, I would take Flynn because at this point, he's the only one that has some upside. Amongst all of his deficiencies, which I don't doubt, Flynn atleast has shown he can produce (which was only in his rookie year but whatever). He's only 23, and I don't think the Rockets or the Blazers really trusted him.
If not him then I would go with Arroyo just based on his very small experience with the team. Doc, KG, and Pierce were impressed with his performance while he was here. Plus he's younger than Bibby and Fisher.
I wouldn't take Bibby or Fisher even if they paid the Celtics to play for them.
HOWEVER, I have to agree with some of what the other posters have said. I would take T-Mac before any of these guys. He's nowhere near the player he was, course we all knew that, but the man is ideal for the fifth guard. He's still a versatile player and wasn't half bad when he played against us last season. His issue is inconsistency, but he'll be the fifth guard so it's not like he'll be a huge piece. The man can play 1-3 and has some good defensive instincts.
Remember, it's the FIFTH guard. The one that's not going to get much time on the team. We already have a great guard rotation in Rondo, Bradley, Terry, and Lee. It's not like we are desperate for more talent.