|Art: Alex Levin|
The highest aspect of the Jewish soul - the yechidah - is so sublime that it cannot be contained within the body, and it spreads to a distance of four cubits [approx 6 feet] around a person. The yechidah is also a level of the soul which can never become tarnished, because it is not susceptible to any negative influences.
Thus, when G-d placed the entire Land of Israel within four cubits of Yaakov to stress his future ownership of it, the Land became connected with Yaakov's yechidah, and so too, with the yechidah of every single one of his descendants.
And that is the reason why "it would be as easily conquered by his children", because the Land was associated with a level of the soul which is impervious to any opposition.
Source: Likutei Sichos Lubavitcher Rebbe