Let $x^4 - e_1x^3 + e_2x^2 - e_3x + e_4 = 0$ be the equation with roots $a,b,c,d$. By
Newton's identities, $$\textstyle e_1 = p_1 = 4,\qquad e_2 = \frac12(p_1^2 - p_2) = \frac12(16 - 6) = 5,\qquad e_3 = \frac16(p_1^3 - 3p_1p_2 + 2p_3) = \frac16(64 - 72 + \frac{188}9) = \frac{58}{27}.$$ So the equation is $x^4 - 4x^3 + 5x^2 - \frac{58}{27}x + e_4 = 0$. Since $58$ is close to twice $27$, write the equation as $$x^4 - 4x^3 + 5x^2 - 2x = \tfrac4{27}x - e_4, \\ x(x-2)(x^2 - 2x + 1) = \tfrac4{27}(x-s),$$ where $s$ is a constant. Now look at the graph:
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The roots of the equation are the points where the blue line meets the red curve. By using the slider, you can see that if $s<2$ then the largest root is greater than $2$. But if $s>2$ then the blue line goes lower, and only meets the red curve in two points, which means that two of the roots of the quartic equation are complex. So for the equation to have four real roots in the interval $[0,2]$, $s$ must be equal to $2$. After multiplying by $27$ the equation then becomes $(x-2)(27x^3 - 54x^2 + 27x - 4) = 0$, which factorises as $(3x-1)^2(3x-4)(x-2) = 0$. Therefore the solutions to the system are $\{a,b,c,d\} = \{\frac13,\frac13,\frac43,2\}$ (in any order).