Everything that you need should be available in the 4620L User Guide:
See page 68 for more info on the GPS over Wi-Fi feature.
In a nutshell you need to make sure that your application and computer are setup to recieve the GPS stream that is generated from the MiFi. A specific port needs to be created by the GPS driver. Once the port is created point your app at it and wait to see if it can recieve the GPS signal.
John, could you recommend some free software to try with this feature to confirm that my MiFi settings are correct?
Unfortunately I do not have any free software recommendations that are compatible with this feature of the MiFi 4620L : (. All I know is what I have read in the manual and from other threads on this forum.
I'd suggest doing a web search for "Free NMEA software" and see what pops up. There should be a few options to choose from that are free or have a free trial.