Cutting your SIM card is risky if not done exactly right. I highly recommend that you drop by yout local T-Mobile store (not a reseller) and ask for the micro SIM card. Apart from a "safer" approach, if your SIM card is old then you might not get full benefit of T-Mobiles's 4G services. The new micro SIM card will make sure your Nexus 4 benefits from the new capabilities.
As you are a T-Mobile customer, they will provide one to you for free (done it twice now and so did our daughter)
SIM card is not always free when your're an exiting customer. I got 2 micro SIM cards at two different time at two different tmo stores. One is free and the other one I had to pay $10. You can also call customer service and ask nicely. They may send you one for free.
Anything can go wrong but cutting SIM cards is rocket science. People has been doing that since the iPhone 4 first came out. This is not something that is done with a pair of scissors. You would use a tool that is pretty much error proof. You can buy one of these tool for under $10 or most cell phone shops can cut it for you free of charge.
I hear from some sources that they're supposed to be free to Contract customers, but also have heard that if you're upgrading a device and need a different SIM card you have to pay.
Because there's no official DOC on it, I don't know what the official T-Mobile stance is.
The rule is always ask nicely -- and, if you've gotten it free before, go ahead back to those same peeps to get it free again!
Thank you for reaching out. All of these answers are very helpful, so thank you everyone for helping out.
SIM cards normally cost about $20 in store if you are just picking one up. If there are technical issues, and account notes support it, stores may provide it to you for free. Asking nicely always helps too.
Since there has been no response, I will mark this as answered. If you have any other questions wtt02005, then you can reach back out.
Thanks again everyone!!