You mentioned that you are looking for at least a 4 bedroom so I would recommend for you to search for cluster houses. It is similar to a
condo with facilities, except it is a house. I think there are a few cluster houses around Hougang area (search Kovan MRT) which is on the same line as Woodleigh. You might want to investigate that area.
Do note that it is a very local area, but I think the cluster houses there are of a good size. Having said that, I won't be surprised if there is a big expat population there as well. You should get one of your colleauges to go there on a weekend, so that he/she can have a better idea. It is also 2 train stations away to Woodleigh and its probably one of the more affordable cluster houses you can find in Singapore that doesn't cost an arm and leg. The school is likely able to provide bus services to pick up your son from your place, considering the close distance. There is the occasional hipcup in the train but it is nothing close to a London's tube or Paris' daily train strikes.
SAIS is not within City Center. So, you would have to decide if you want to stay nearer to the City Center (aka your office) or nearer to your son's school.