mmm 4Gb is very small indeed. The notebook is intended for children really .
You'll need to use cut down or lite versions of any software you need like word and spreadsheets etc.
You can use a USB pen drive to store all your digital images and MP3 files etc to free up your drive space. They are only about £10 in Asda or wherever.
If you use an external modem you'll still need to install drivers . Every bit of hardware requires drivers in order for the laptop to use it. naturally you will need Ubuntu drivers not Windows .
If I were you I would dump ubuntu and install Win 2000 or Win xp You can download Winlite which is a program that can reduce the size of Windows down to just a few hundred Mb. Ubuntu is for geeks and is a complete nightmare to get working properly .
Google is your friend .
http://hehe2.net/linuxhowto/howto-your-perfect-ubuntu-on-your-perfect-eee-pc/
I don't mean to be insulting but with you having such a minimal experience of such a vast subject It would be an impossible task trying to sort this problem on a forum. It would require hours of work to install everything and configure it all to get a useable laptop.
To give you an example it takes me at least 6 hours to format a disk partition it and install windows, office suite,Image and Music editing software, DVD player,All your drivers . Then setup internet service provider ISP email account, Then there is all the vital stuff like antivirus, firewall, Adobe PDF , Winzip , CD ripper MP3 converter , printer software and drivers etc etc
