Not just children under 4. There is a study going around that found 12-15 year olds are ~5X more likely to be hospitalized due to vaccine induced myocarditis than hospitalized due to covid.
"boys aged 12 to 15, with no underlying medical conditions, are four to six times more likely to be diagnosed with vaccine-related myocarditis than ending up in hospital with Covid"
"About 86% of the boys affected required some hospital care"