Probably something like what I suggested above. I figure the average Divine Shield user is 10+ usages per day, and is probably getting between +8-10 AC, typically.
So, keep Shadow Evade capped at +4, but extend the uses and duration to be somewhat similar to Divine Shield, but perhaps a little longer/more uses due to the lower AC buff.
How about these possible scenarios:
Duration: 1 round/level of Shadowdancer
Usage: 1/day per Shadowdancer level
or
Duration: 2 rounds/level of Shadowdancer
Usage: 3x day
or
Duration 2 rounds/level of Shadowdancer
Usage: 1/day per 2 Shadowdancer levels
Any of those sound appealing?