4 pc. hands down for BM. It is THE best set bonus that ever existed ever for the spec. It is amazing. For more definitive values use this (i'm surprised you're not familiar with it already): http://www.femaledwarf.com/
Load your toon and then substitute the off pieces for the tier pieces.
yup around 497-499 ilvl range 300 agi is ~1200 DPS whereas the 4pc for BM gives about ~2500 DPS. More than double. So unless you gain ~600 agi atleast, it's not worth it to lose the 4pc. Again, as BM.
When you switch to SV for AoE/Multi Target fights, i'd go with the off pieces for the extra 300 agi. Since you won't be able to fully utilize the 4 pc in a non single target fight, so the actual value of the 4 pc would drop even more. 300 agi would benefit you a lot more on aoe etc.