This was done for "Stability and performance improvements" over v3.9 that saw problems with the Search function causing not only extreme slowdowns but freezes.
At the Kindle Forums, Kindle Universe contributor Tom Semple points out, re the dictionary function:
' In terms of the update, I cannot use a Russian-English dictionary I have with the 'Bring Your Own Dictionary' feature. The app detects that it IS a dictionary, and lists it along with the other dictionaries, but when I select it, no definition is returned.
The same dictionary and Russian text works perfectly on my Paperwhite. Seems like a bug. '
[ AB Note: Tom adds something below that he noticed with Dropbox (which is
not an Amazon appstore app but is very popular.]
' I don't know if this is related to the update, and haven't tried it for awhile, but I used to be able to open .mobi files from Dropbox (it should offer Kindle as one of the Open In choices).
This does not work on my iPod Touch, but does work on iPad. '
Current Kindle Models for reference, plus free-ebook search links.