What would be a good medal for DB 2 Max if your 0.2 Kairi already has DB4?
It is actually debatable whether it is worth it to replace a DB4 with DB2Max. DB4 reduces damage to 12.5% some of the time, while DB2Max would reduce damage to 33.3% all of the time.
Personally, I would only use Defense Boosts for medals that you're expecting to use to tank damage, so the two recent Union Cross defense medals would be ideal, but I already put a DB4 on both of them.
So... I think it would be best to wait and see, at least until you can get your hands on a spare DB4.
EDIT: Actually, based on how probability works, it may indeed be best to replace a DB4 with DB2Max. Let's say that an enemy will deal 5 hits to you that will do 3,000 damage each. DB4 will, on average, activate twice as you get hit, while DB2Max will activate for each hit. So, here's some mathing to go along with it:
With DB4: 375+375+3000+3000+3000 = 9,750 damage
With DB2Max: 1000+1000+1000+1000+1000 = 5,000 damage
Even if you have two DB4 medals equipped, the average amount of activations for 5 hits will be around three, so you still take over 6,000 damage. It seems that a consistent Defense Boost is better for virtually every situation.