I am running BT4 final on a virtual box machine and am running windows 7 as the normal operating system. I don't have networking equipment: Router, etc.
I am wondering if it is possible (and how) to run a virtual network on my computer to practice on without having to buy a new router and all that networking equipment.
There is no spoon.
Some what tough request, however here are are a couple good articles to help get you started.
Virtual router labs
Running a Virtual Router & Firewall inside VMware ESX with Vyatta
For more try typing virtual and network or router etc into google.
Do you actually want virtualized routers in your test environment, or are you asking whether it's possible to exchange network traffic between VMs (it is).