Context : Understanding the nature of the power source in luminous infrared galaxies ( LIRG ) is difficult due to their extreme obscuration .
Observations at radio and mm wavelengths can penetrate large columns of dust and gas and provide unique insights into the properties of the compact obscured nuclei of LIRGs .

Aims : The aim of this study is to constrain the dynamics , structure and feeding of the compact nucleus of NGC 4418 , and to reveal the nature of the main hidden power source : starburst or AGN .

Methods : We obtained high spatial resolution observations of NGC 4418 at 1.4 and 5 GHz with MERLIN , and at 230 and 270 GHz with the SMA very extended array .
We use the continuum morphology and flux density to estimate the size of the emitting region , the star formation rate and the dust temperature .
Emission lines are used to study kinematics through position-velocity diagrams .
Molecular emission is studied with population diagrams and by fitting an LTE synthetic spectrum .

Results : We detect bright 1 mm line emission from CO , HC _ { 3 } N , HNC and C ^ { 34 } S and 1.4 GHz absorption from HI .
The CO 2–1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s ^ { -1 } .
We detect vibrationally excited HC _ { 3 } N and HNC , with T _ { vib } \sim 300 K. Molecular excitation is consistent with a layered temperature structure , with three main components at 80 , 160 and 300 K. For the hot component we estimate a source size of less than 5 pc .
The nuclear molecular gas surface density of 10 ^ { 4 } M _ { \odot } pc ^ { -2 } is extremely high and similar to that found in the ultra-luminous infrared galaxy ( ULIRG ) Arp220 .

Conclusions : Our observations confirm the presence of a molecular and atomic in-flow , previously suggested by Herschel observations , which is feeding the activity in the center of NGC 4418 .
Molecular excitation confirms the presence of a very compact , hot dusty core .
If a starburst is responsible for the observed IR flux , this has to be at least as extreme as the one in the ULIRG Arp 220 , with an age of 3-10 Myr and a star formation rate > 10 M _ { \odot } yr ^ { -1 } .
If an AGN is present , it must be extremely Compton-thick .