Your hard drive cannot be 4GB, that must be your memory (RAM) size. Windows 7 32bit takes I believe close to if not over 32GB of space and Windows 7 64bit requires a bit more.
What is your hard drive capacity?
This is required to recommend partition sizes. Please also indicate how many partitions and their purpose.
On a typical 500GB or larger hard drive, I would partition your system drive (operating system and programs) as 120GB during the installation and later, after Windows it up, partition the rest for your personal files.
Using the instructions in your manual, enter your BIOS set up (it may be Delete key, an F key, or another key during boot up). In your BIOS set up, set your boot order to your DVD drive first and your hard drive second (for those still using floppies, it is Floppy drive, DVD drive, then hard drive), save changes and exit.
Then you put in the installation DVD and 'Hit any Key to Boot from CD/DVD' when asked to during start up. This will start you into your Install DVD. You will get some options, If you do, you may need to choose Custom and then Partition. Then you need to delete the current partition(s), create a new partition out of the free space, and partition it to NTSF. Then you can proceed with the clean install.
Clean install simply means to install it on a new partition rather than over the existing operating system.
If you have less than 4GB RAM, don't bother. If you plan on getting a total of greater than 4GB RAM you need the 64bit OS. If you plan on running programs that use a lot of RAM (Photoshop, 3D CAD, some programs for work on images and videos, databases and Excel where there are entries of around 60,000 entries or more, etc.). In other words, for general computing, it probably is not worth the bother since you will probably never need it or the additional memory.
You have to be a little bit crazy to keep you from going insane.