I called Google device support and they referred me to this Youtube http://www.youtube.com/watch?v=eb0iCtG4T6I
It advises installing a phoneutil.apk on the phone and QPST on a Windows computer. I verified that the Samsung USB drivers are installed. But where I reach the step where I enter *#0808# on the phone to get to the menu where I can select DM+MODEM+ADB the procedure fails. Entering *#0808# gets me "USSD code running..." shown briefly before showing "Unable to process request" and an OK button. Without this step, the Samsung USB Modem drivers did not show under Ports in Device Manager when the phone is connected via USB. I have used the original S4 USB cord. No com ports show with 0 in the Phone column in QPST and I am led to believe that I can't install the 337_AWS.qcn file to get the additional bands.
My bootloader *is* unlocked (and the phone is rooted). I read this thread which refers to *#0808# as well. Ihttp://forum.xda-developers.com/showthread.php?t=2370466
I even tried the older *#7284# with the same result.