Key Stage 4 is the last compulsory stage in your child’s education, covering ages 14-16 and years 10 and 11, the final two years of secondary education.
During this time, your child will study a range of core subjects along with a number of optional subjects they would have selected at the end of Key Stage 3. At the end of Key Stage 4 teens will sit their end of secondary school exams. These will most likely be GCSEs (General Certificates of Secondary Education), though a range of other qualifications are gaining popularity, such as NVQs (National Vocational Qualifications). The final exams will bring together everything your child has learned in secondary school and will help to determine their next steps, be it further education, employment or an alternative.
Source Reference: What are the Key Stages in the National Curriculum? - From WHSmith