It means they are combining the 2x lossless optical zoom with 2x lossless digital zoom for a 4x total zoom. It is only possible in FHD (Full HD, aka 1080p) because to obtain lossless digital zoom they crop the image of the 4K sensor (vs downscaling from 4K to FHD). When you are shooting in 4K, you only have the 2x optical lossless zoom available because you are using (mostly) all the pixels on the sensor.