1. Well if you port the number wont you end up phoneless until you get the nexus?
2. I'm sure you'll have to call T-Mobile.
3. The Nexus 4 uses a microsim, so if you ordered that, all you would need is a sim adapter and you can pop into the prepaid to test out T-Mobile. I would say just wait until your Nexus 4 gets in.
4. Google Voice seems nice, you can link all your phone numbers together to that number so they all ring at once. I believe SMS is free. Thats all I remember for now I'm not very knowledgeable on the pros and cons so check it out here.
Google Voice - Features