Context : The galaxy NGC 4418 contains one of the most compact obscured nuclei within a luminous infrared galaxy ( LIRG ) in the nearby Universe .
This nucleus contains a rich molecular gas environment and an unusually high ratio of infrared to radio luminosity ( q-factor ) .
The compact nucleus is powered by either a compact starburst or an active galactic nucleus ( AGN ) .

Aims : The aim of this study is to constrain the nature of the nuclear region ( starburst or AGN ) within NGC 4418 via very-high-resolution radio imaging .

Methods : Archival data from radio observations using the European Very Long Baseline Interferometry Network ( EVN ) and Multi-Element Radio Linked Interferometer Network ( MERLIN ) interferometers are imaged .
Sizes and flux densities are obtained by fitting Gaussian intensity distributions to the image .
The average spectral index of the compact radio emission is estimated from measurements at 1.4 GHz and 5.0 GHz .

Results : The nuclear structure of NGC 4418 visible with EVN and MERLIN consists of eight compact ( ¡49 mas i.e .
¡8 pc ) features spread within a region of 250 mas , i.e .
41 pc .
We derive an inverted spectral index \alpha \geq 0.7 ( S _ { \nu } \propto \nu ^ { \alpha } ) for the compact radio emission .

Conclusions : Brightness temperatures > 10 ^ { 4.8 } K indicate that these compact features can not be HII-regions .
The complex morphology and inverted spectrum of the eight detected compact features is evidence against the hypothesis that an AGN alone is powering the nucleus of NGC 4418 .
The compact features could be super star clusters ( SSCs ) with intense star formation , and their associated free-free absorption could then naturally explain both their inverted radio spectrum and the low radio to IR ratio of the nucleus .
The required star formation area density is extreme , however , and close to the limit of what can be observed in a well-mixed thermal/non-thermal plasma produced by star-formation , and is also close to the limit of what can be physically sustained .