This seems to be the only new things:
Two color bag
More available color options
Velcro on the sides
Two velcro patches
NEW: Removable/Replaceable Backstraps
NEW: 4 short pencil pockets (really....how many pencils do I really need to carry? I'd rather have one or two pockets for them and carry extras in a larger, side pocket)
I'm not sure what is new about these two things....I think the NEW might be left over from the V3 bag description.
2 Large integrated 32oz Water Bottle holders NEW
haul backstrap clinch side buckles to bring the load closer to your upper back NEW
I wish they had a trade-in policy. I really like the V3 over the V2 (which I have). When the V3 came out, they also offered a less expensive version without the seat (you used the seat from your existing bag). I don't care for where the velcro is on V4 - I have towels and other things hanging on the sides, so no one would ever see the patches. And the patches they provide aren't ones I would really want on my bag.
But...if I didn't already have a RidgeRoller, I'd be getting the V4. My only concern would be the removable backstraps...how well are they attached? Will they come unattached while it's on my back? Or when I'm lifting it off the ground?