If you look closely you will see, that actually only the
-doc packages contain a version 4 in their package name, while the packages that ship the actual compiled libcurl library are still at version 3.
For a discussion on Debian about this, see this link: https://lists.debian.org/debian-release/2007/04/msg00257.html
If I understand it right, there was a change in the API but not the ABI, so the binary packages were left at version 3, while the development files needed a version bump.
It also means that for example
libcurl4-openssl-dev is the corresponding development package for
libcurl3 despite the different version in the package name. You can see that if you take a look at
libcurl3-dev (which is a virtual package and points to
No current or candidate version found for libcurl3-dev
State: not a real package
Provided by: libcurl4-openssl-dev (7.47.0-1ubuntu2),
The above snippet was taken on Ubuntu 16.04 but this will probably look similar on 14.04.