Both dependencies are resolved (as answered by yum provides --enablerepo=centosplus ...) by postgresql-libs (for libpq.so.4) and mysql (for libmysqlclient.so.15) ... but it seems you've excluded some packages from your centosplus repo config file .... Be aware that the newer postfix package needs mysql5 .... consider this prior to upgrade !
Yes. More info at: http://bugs.centos.org/view.php?id=2195
And thanks to all CentOS development team to comply with community needs.
:)