If you want to fully open a cabinet door next to something deeper than the door, yes, you do need filler -- but not 4" of filler! 1.5" to 2" would probably be sufficient.
Another option: Filler Pullouts
Filler pullouts utilize the space b/w two cabinets (or end panels). The pullout is installed on the walls of the adjacent cabinets. Rev-A-Shelf has a selection of base cabinet, upper/wall cabinet, and tall cabinet filler pullouts. They range from 3" to 6" for upper/wall cabinets, 6" for tall filler pullouts, and 3" to 9" for base filler pullouts. Rev-A-Shelf isn't the only one that has them and you can probably find them cheaper elsewhere. I'm just posting the link so you can see possibilities.
Base cabinet filler pullouts: https://www.rev-a-shelf.com/rev-pages/product-listing.aspx?CategoryFilterID=47&SubcategoryFilterID=53
Upper/Wall cabinet filler pullouts: https://www.rev-a-shelf.com/rev-pages/product-listing.aspx?CategoryFilterID=66&SubcategoryFilterID=71
Tall/Pantry filler pullouts: https://www.rev-a-shelf.com/rev-pages/product-listing.aspx?CategoryFilterID=60&SubcategoryFilterID=62
Upper/Wall cabinet filler pullout next to the a cabinet deeper than the pullout: https://www.rev-a-shelf.com/showProduct.aspx?SEName=filler-pullout-organizer-with-stainless-panel-wall-accessories&ProductID=251
Look at all the options so you can see possibilities. In particular, note the upper/wall filler pullout that is up against a cabinet deeper than it.
The nice thing about pullouts is that you need very little filler to have full access. (Since you will have frameless cabinets, you will probably have to have 1/2" to 1" or so.)
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Regarding the corner...do not use a blind corner -- they're one of the worst uses of the space. Instead, consider voiding out the corner and installing wider cabinets on each side of the corner.
You do need 3" b/w the voided corner and the cabinet next to it to allow all cabinets to open on both sides without hitting each other. Here's an example of what I'm talking about:
Note: The examples below were worked up for someone who was replacing the corner with a 27"D cabinet turned 90 degrees to face outside the Kitchen. This can only be done with a peninsula that's open for the full length. However, the 3" of filler needed on each side of the corner applies in both yours and the example's situation.
The right way:
Wrong Way:
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Upper Corner...normally, I would suggest using an "Easy Reach" cabinet in the upper corner instead of a diagonal; unfortunately, IKEA does not offer an "Easy Reach". ("Easy Reach" is a cabinet that is 24" x 24" but with 90-degree angle instead of the diagonal. They're easier to use and they give you more "face space" in the corner.) If you have expert DIY skills, you might be able to hack an IKEA cabinet to make one. Some hacks can be done with average DIY skills, but I think this particular hack would require expert skills.
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Different approach to shelves in shower.
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