MPEG-4
MPEG-4 is a family of standards for compressing audio and video. DAB+ uses its audio part, AAC, for the sound.
MPEG-4 is a set of standards for compressing and storing audio and video. The part that matters for radio is the audio coding, which includes the AAC method.
dabplus.ch describes DAB+ as using MPEG-4 AAC+ coding, which is the reason it carries more programmes per frequency block than the original DAB.
The term appears on spec sheets and in explanations of DAB+. A radio that states MPEG-4 AAC+ support is describing the decoder that DAB+ needs. The reliable test for a buyer remains the DAB+ label on the device and in the manual.