I just noticed that 2016 has a scroll bar on the drop down, 2013 doesn't. Is it possible that the "right" B4 is higher up on the list?
In 2013 the "more paper sizes..." option displays the Page Setup dialog > Paper tab. In there I have very limited size options.
That is interesting, it seems that in 2016 MS has redefined the B4 Standard. Here is what I see in 2013:

Which is correct? ... Depends on your location.
I googled several sites, they all agreed with wiki, suggesting MS was being "creative". Then I found this (very old) article:
https://msdn.microsoft.com/en-us/library/windows/desktop/dd319099%28v=vs.85%29.aspx?f=255&MSPPError=-2147217396
There B4 is listed same as 2016. BUT, it includes the qualification "JIS". That is a Japanese standard. And when I looked farther down the page, there was also B4 ISO, which is European and the same as Wiki.
SO ...
try clicking on the "More Paper Sizes ..." option
to see if there is also an ISO B4 size listed farther down.
Alternate work around ...
Earlier versions of Word were very tightly connected to the selected Printer driver. It seems to becoming less tightly connected, but maybe this is one setting that is still associated with the Printer driver. Try selecting a different printer to see if
that changes the options.
Please VOTE:
MS has created some "UserVoice" sites for us to submit change and feature requests. I have created a suggestion based on your question. Please vote for it in the hope that MS will eventually act on it
https://word.uservoice.com/forums/304924-word-for-windows-desktop-application/suggestions/11447529-word-2016-paper-sizes-need-clarification
"UserVoice.com" - MS Suggestion box sites
If you have an (other) feature suggestions to make for various MS products, here is a longer collection of MS UserVoice.com sites:
Here are links to some of my personal “favorite” suggestions
·
Recently Used List Sort order: Date vs Alpha
https://word.uservoice.com/forums/304924-word-for-windows-desktop-application/suggestions/10137741-recently-used-list-sort-order-date-vs-alpha
·
Petition to MS President to allow Windows Home users control Windows Updates
https://www.change.org/p/satya-nadella-microsoft-what-computer-users-want-changed-in-windows-10 (The matching UserVoice site has been
discontinued by MS)
·
2016 QAT is too Spaced OUT! -
http://office.uservoice.com/forums/285186-general/suggestions/9906966-office-2016-quick-access-toolbar-too-wide
·
Don’t want Office 2016 tied to Userid -
https://office.uservoice.com/forums/285186-general/suggestions/10517283-allow-activation-without-the-need-of-a-ms-mail-acc
·
Visio 365 monthly specifically for Consumer licenses -
https://office.uservoice.com/forums/285186-general/suggestions/10936428-visio365-monthly-for-consumer-365-users-and-oth
·
Recreate the single text margin around page like 2010 and earlier -
http://word.uservoice.com/forums/304924-word-for-windows-desktop-application/suggestions/9253599-bring-back-classic-text-boundaries
·
Add mind mapping to OneNote -
https://onenote.uservoice.com/forums/327186-onenote-for-windows-windows-phone/suggestions/6160979-mindmap-functionality?page=4&per_page=20
·
Custom installation from Office 365 (2016) / Return MSI installer option -
https://office365.uservoice.com/forums/264636-general/suggestions/9939513-custom-installation-from-office-365-2016
·
Word 2016 Style Pane bug -mouse pointer wrong
https://word.uservoice.com/forums/304924-word-for-windows-desktop-application/suggestions/11196345-word-2016-style-pane-corrupt-or-buggy-after-update
·
Add PowerPivot back to Access (and to Excel in all bundles)
https://access.uservoice.com/forums/319956-access-desktop/suggestions/10284183-add-pivot-charts-back-to-access
·
Add BI/PowerPivot back to Excel in all bundles -
https://excel.uservoice.com/forums/304921-excel-for-windows-desktop-application/suggestions/9018823-make-the-power-bi-family-powerpivot-powerquery