Where does it say you can only get 8 at a time, I know you can in current system but in the current system you can get 8 Field also so its like 16, this says you get 4 spread across all, so where does the 8 factor in?

My assumption was that you get 4 "Slots" every 12 hours that you can fill with any type of leve. If you wait 36 hours without doing any leves you now have 12 slots which you can then collect 12 battleleves or 12 fieldleves, up to you, you can grab 12 in a single instance and then go do them in a single instance and you dont need to come back.

To prevent people getting up to 99 and then blasting through, the system would only provide you 1 unique leve every time, so for example if you've collected "Dinner for Dunesfolk" the next reset, that leve does not show up and another does, when you've collected all for that category it no long has any and you pick from another.

I also think this due to the 99 build up.

For example:
00 : Collect 4 rank 40 leves
12 : Collect 4 more rank 40 leves (all unique)
36 : Collect 4 more rank 40 leves
48 : Collect 4 more rank 40 leves
60 : collect 4 more rank 40 leves.

Now after 2/half days you have 20 leves to go through in 1 night.

Just my guess.