If you have 4GB RAM, you probably won't see any difference between 32-bit and 64-bit versions of Windows 7. Older peripherals may not have 64-bit drivers.
I'm running 64-bit Windows on one machine with 12GB RAM because I have a couple of applications that work better on 64 bits. It's fast and relaxed. On your machine, L4D will probably slow down a little.
32-bit Windows 7 can address 4GB RAM and will actually have access to around 3.5GB, depending on your video card.
The maximum memory for the various 64-bit version of Windows 7 are as follows, unless anything's changed recently:
Starter: 8GB
Home Basic: 8GB
Home Premium: 16GB
Professional: 192GB
Enterprise: 192GB
Ultimate: 192GB
Ignoring the Atom, the last consumer desktop 32-bit Intel CPU, the original P4, hit the market in early 2004. CPUs since 2005 have been 64 bits.