4. Suppose $a , b , c$ are I A.P. and $a ^ { 2 } , b ^ { 2 } , c ^ { 2 }$ are in G.P. If $\mathrm { a } < \mathrm { b } < \mathrm { c }$ and $a + b + c = 3 / 2$, then the value of $a$ is\\
(A) $\quad 1 / 2 \sqrt { } 2$\\
(B) $1 / 2 \sqrt { } 3$\\
(C) $\quad 1 / 2 - 1 / \sqrt { } 3$\\
(D) $\quad 1 / 2 - 1 / \sqrt { } 3$\\