Answer
The required value of $D$ would be less than the value found in part (a).
Work Step by Step
If particles 3 and 4 were moved closer to the x axis but maintained their symmetry about that axis, the magnitude of the force on particle 1 due to particles 2 and 3 would increase since the distance would decrease.
Also, the horizontal component of the force would be a larger portion of this force. Therefore, the net horizontal component of the force on particle 1 due to particles 3 and 4 would increase.
In order for the net force on particle 1 to be zero, the force on particle 1 due to particle 2 would also need to increase.
Therefore, the required value of $D$ would be less than the value found in part (a).
