Currently, dev-qt/designer:4 installs /usr/lib64/qt4/libQtUiTools.a, while all other Qt libraries are installed as *.so's. I suspect this isn't intended? Note: Other distributions provide libQtUiTools as a shared library.
It pretty much *is* intended. uic interfaces were by upstream choice not meant to be stable - just recall the first few lines of an uic generated file.