pyqt4 & qt license

[pitsakis at gmail.com]

hello,

i want to develop a GUI application that will be sold. 
i want to use pyqt4. 
can i download and use the GPL 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