Is it permissible to abort my wife’s pregnancy, which is less than a month old, due to the presence of four children close in age and a work assignment that requires travel?

The basic principle is that abortion is forbidden, unless the continuation of the pregnancy poses a danger to the mother's life. However, if the pregnancy is less than forty days old, and the situation is as you described, with the children's ages being close, then there is no impediment to aborting this pregnancy to avert hardship.