pyqt4 & qt license

[pitsakis at gmail.com]

i want to develop a GUI application that will be sold.
i want to use pyqt4.
can i download and use the GML version during the development and then buy the commercial verion beofore i distribute the application ?

commercial version means LGPL? i think i am comfused so please someone clarify on versions and licences.

somewhere i also noticed that there is a qt license ? will i need this also ?

Thank you in advance