ok. medical hat on how.. lol

the 4yo vacc is diptheria tetnus and pertusus. (dtpa) this will last up to 5 years, but as it is given in such a fresh immune system they will only gaunrentee it for 4,
so technically you do have 12 months to get this one done. as final booster shot is givenm at 6 months....
the measels mumps and rubella (mmr) will start to wain if not given the booster and if delayed to long will not come back to a "full" coverage, but there will be coverage.

then the inactivated poliomyelitis ( ipv) is the same as the MMR and needs to be as close as possible to the due date.

i personally and professionally wouldnt suggest delaying longer than 6 months.

pre school can not exclude you on a day to day basis but if there is an outbreak of any illness which is covered in the 4yo immunisations you must keep DS home. as they will not take responsibility for him while he is susceptable to the ifection with a "lower" immunity than the other children.


HTH