Select the images you want to geoencode, switch to the Map module, choose Map/Tracklog/Load Tracking (Choose the tracklog file), and choose Map/Tracklog/Auto-tag Photos. I don't believe it's absolutely necessary to download the tracks first. Instead you can connect the GPS device in mass storage mode and download directly to LR4. Edit: I checked my Garmin and it doesn't seem to have a way to switch to mass storage mode. So you need Garmin's software or something like Easy GPS or Geosetter to download tracks and save them as .gpx files.
I still prefer using Jeffrey Friedl's Geoencoding Support plug-in to do the encoding and Easy GPS to download and save the tracks from my Garmin eTrex Vista HCx.