Loveforcanada, you have written another post about this issue. Please don't start multiple posts for the same issue.
You cannot claim to be in common-law relationship since she has only been here for 4 months. If you think she will be granted the extension to explore this relationship, go ahead and submit it. I hope you are right. I only know what I have been told. I spoke with a lawyer before I got married. He adviced me to get married abroad because CIC often denies sponsorship to people who came to Canada under the pretence of "visiting" and then get married here or get into common-law relationships.
You should consider this: If you submit "in-land" spouse or common-law sponsorship and it is denied, YOU CANNOT APPEAL. However, if you submit "out-land" sponsorship and it is denied, you can immideatly appeal and that appeal will be dealt with by another department.