No, I went back to the beginning of the thread. I thought you had 2 unlimited data lines, but you don't, you have one unlimited data line and one 4GB data line. So what you need to do is use the 4GB data line's upgrade. Then switch the new phone to your unlimited data line by putting the 4GB data line's phone back on the 4GB data line. Then when your wife is ready for an upgrade, transfer your unlimited data line's upgrade to her line.
I had 2 unlimited data lines so I had no option but to add a line. As long as there is a tiered data line on the account, this will work. But if the tiered data line has an upgrade available, you can't transfer an upgrade to it. You have to use the tiered data line's upgrade first then switch the new phone to the unlimited data line. Then when the person on the tiered data line wants a new phone, that is when you do the transfer of the upgrade from the unlimited data line to the tiered data line.
There is absolutely no worry at all of losing unlimited data if you use the tiered data line's upgrade to upgrade and then switch the new phone to the unlimited data line.