This paper presents results on four radio pulsars observed at Arecibo. Three pulsars have been discovered as new mode switching pulsars. In two of them the switching effect is correlated with a dramatic change of the amplitude of the core component, suggesting that the change occurs not only in the location of the emission region, but also in the efficiency of the emission mechanism. In another pulsar evidence is presented for the existence of a third (possibly a core) component which is hidden between the two conal components of the average profile. Previously unknown drifting subpulses and at least two new components have been found in one of the three mode switching pulsars.