If you plan on using your old Verizon number with your Nexus 4, don't bother porting to Google Voice, and there's no need to contact Verizon. T-Mobile will handle everything for you. All you have to do is supply them with the info noted above.
Also, note that you may get two bills after you cancel your Verizon account, depending on where you are in your billing cycle. Your old Verizon contract is post-paid, meaning you're billed at the end of the use period. If you cancel on or near your billing date, your next bill may not reflect the ETF as the bill will already be in processing. The ETF will come on a later bill. On the other hand, if you cancel somewhere in the middle, your bill may include everything (ETF and pro-rated month of use).
It's not as complicated as it may sound, and is actually quite painless. You may experience a period where you'll have service on both your old and new phones, but it will be temporary and will clear up once the transfer is complete.