User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-4682548-20130322055408/@comment-1837917-20130322074437

@Monstar Hunter; sounds like:

a. You simply haven't moved the Vivid Feather from your in-quest items to your Inventory Box,

or,

b. When you go over the limit of 3 pages, a 4th page is created so you don't have to worry too much about getting rid of items mid-quest to accomodate for newer items.

If it turns out to be the second one, I'd simply say to reduce the amount of items you're taking with you on your quest :) I'm not certain, but I think that once you finish a quest with that 4th page, it means those items immediately transfer to the inventory. Sounds like it.