If he's already 4, he needs to be in reception class at school, not nursery. You need to apply for a reception place at the school your eldest will be at. You can defer his entry, but only within the academic year.
We just moved here from the States. My newly turned 4 year old (June birthday) attended one year of 1/2 day preschool this past year. He would have done the same come September but we moved to London for 2 years. Am I able to have him do 1/2 day "nursery" come September or defer his entry until January? I am unfamiliar with the system for young 4 year olds. My daughter (age 7) will be attending either Holy Cross or Sulivan. Thanks in advance!