There have been some reports on people with problems to get PyQt4 working properly in Pydev... This is because depending on the version of PyQt4 it has a major bug that will crash a Python interpreter (this was confirmed in version 4.5.4 -- haven't checked on other versions).
To know if you have a version with this problem you can check it by running the code below in an interactive session:
>>> from PyQt4.QtCore import *
>>> import inspect
>>> inspect.ismodule(QSignalMapper) <-- this will make the interpreter
If that's your problem, right now you have to fix Pydev by editing
eclipse/plugins/org.python.pydev/PySrc/importsTipper.py at line 154,
"for d in dirComps:"
for d in dirComps:
if d == 'QSignalMapper':
Or get another PyQt4 version.