Hi folks!
This was just a personal itch I scratched, but maybe someone else has use for the outcome nonetheless.
It's based on the bookmarks version of ipdf, and I did an effort to keep usage changes nominal (see below

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Basically,
1) I was missing the ZoomWidth feature in Landscape Continous mode <- The ZoomToFit icon now toggles between ZoomPage and ZoomWidth
[known issue: ZoomWidth needs to be enabled _before_ turning on Continous in Portrait mode - otherwise it has no effect]
2) I was missing a zoom mode splitting up the page into 4 parts in Portrait mode <- introduced a ZoomPart mode to Portrait mode (exclusively) that scales to 1/4 page + Epsilon (to avoid problems with cut lines), and that allows flipping between the quarters by pressing the Up/Down buttons
[known issue: produces funny zoom states in Portrait Continous mode - simply don't use it (easy to revert by disabling ZoomPart again, if it happens)]
Since all buttons were used, I turned [dot] into a modifier, and changed the Up/Down-Button functions when ZoomPart is enabled:
[dot] [dot] <- the new [dot]

[dot] [longUp] <- toggle ZoomPart mode (only in Portrait mode)
When in ZoomPart:
[Up] - Go to Part + 1 [upleft->downleft->upright->downright->upleft]
[UpLong] - Go to Part + 2 (effectively skip sidewards)
[Down] - Go to Part - 1
[DownLong] - Go to Part -2
Mind that using the ZoomToFit icon does not disable ZoomPart (so you may use it to go to fullscreen, and then go back via Up/Down

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Disclaimer: Though I see no reason why it should happen, I won't accept resposibility for any things damaged/corrupted/lost due to this software. This is not a bulletproof tested, and if you fear it might eat your newborn babies, check the source and compile it for yourself.
Cheers!