How to get better range on my 2 x XBEE 900HP S3B peer-to-peer communication?

I have two XBEE 900HP S3B modules with 2dBi dipole antennas on them. I have accomplished communication between them when working in my house, but I need to get communication of about 3 or 4 km (1.8 to 2.5 miles) in urban places. This cards have a maximum range of about 28 miles with LOS using high gain antennas.
So I want to know what do you recommend me to get this working on urban areas with a range of 4 km. It is possible to change the transmission power to its maximum? How much gain is “high-gain” for an antenna? What other changes must I do to get this working?
Thank you very much

I would suggest that you look over the following articles as it will help you to understand what you need to do.

Simply put, you are going to need to obtain RF Line Of Sight before you are going to get this kind of distance.