Do you think a 30l blue barrel would be big enough for 4 people on a 6 day trip? I've only tripped with 2 people & this barrel. Considering buying another small bear vault but I don't really want to. Just thinking it might be a task sifting through one barrel for snacks everyday. What's your thoughts, what do you use for a group of 4?

It all comes down to what kind of food are you bringing, right? As you said, it could be a task sifting thru it daily to find the things you need.... More likely you'll be unpacking & repacking daily unless you devise a system to pack it in layers that also don't consume useful space. A 10 gallon barrel, if you could find one, would be better.

I will be using a 30L barrel this year and 2 Ursack AllMitey's. I have (3) BV 500's and a Bear Assault canister. They are good for up to 3 people for a few days but once you get to 4 people for more than 3 days, they become bulky. A blue barrel, CCS Barrel Quad Pack, and 2 Ursacks weigh exactly half what the bear canisters and 2 small packs to carry them weigh. I do love the BV 500's though. They can be used for chairs, you can easily see what's in them, and you don't have to worry about your food. I use 100% freeze-dried food except for snacks.

We use a 30L barrel and a large BV500 canister. This contains everything for about 10 days worth of food for 2 people. This does not count the first days lunch or snacks which are in a day pack. I would imagine you will probably need two 30 L barrels. Plus we pack very light with food for my wife and I. As we do a lot of our own dehydrating of meals and use just a couple of freeze dried meals.
To organize things in the barrels gets some different colored stuff sacks that aren't too large. We have one for all breakfast items, one for lunches and dinner and one for snacks and drink mixes. Our lunch and snacks for the day are moved into the Bear Vault each day to stay protected plus some extra meals are in there in case the blue barrel gets trashed by a bear.

I'm not familiar with the blue barrels, but spent some time whittling down the bulk of my food and packaging until I can get 8 days food (1 person) in a BearVault BV500, which is listed as 11.5 liters for comparison. When it has 8 days it's stuffed full in there, not just loosely packed. I don't know how bulky yours will be.

I got almost 9 days in one time, but you'll be packing the equivalent of 24 days for a solo. I think it would be very hard and you'll probably need just a little more space. I also don't pack the first day's lunch/snacks in it; I just carry them in my pocket.