The aim of this population study was to examine the severity and prevalence of co-occurring developmental delays in 4-year-old children, the rate of overlapping problems, and sex differences. A sample of 434 children (196 males, 238 females; mean age 4 years 3 months, SD 1 month) were administered the ‘Lene’ test: a comprehensive neurodevelopmental screening test. Results suggest that co-occurrence of attention-behavioural, motor-perceptual, and language delays occurring in school-aged children could already be detected at the age of 4 years. Isolated delays were usually mild, but co-occurring difficulties were mostly moderate or severe. Overlap between developmental delays depended on the severity of the problems. It emerged that males had more severe and more often co-occurring problems than females. Co-occurrence of developmental delays as a risk factor at the early stage of development is discussed.