Among $6$ persons U, V, W, X, Y and Z, it is known that: U is taller than at least $3$ persons. W is taller than V, who is taller than exactly $1$ person. X is not shorter than W. Y is taller than U and X is shorter than U. Who is 4th tallest among them?

Let the positions from 1st (tallest) to 6th (shortest) be represented as ${\text{P}}_1>{\text{P}}_2>{\text{P}}_3>{\text{P}}_4>{\text{P}}_5>{\text{P}}_6$.
* "V is taller than exactly 1 person" $⟹$ V is at ${\text{P}}_5$.
* Thus, ${\text{P}}_6$ is the only person shorter than V.
* "W is taller than V" $⟹$ W must be in one of the positions $\{{\text{P}}_1,{\text{P}}_2,{\text{P}}_3,{\text{P}}_4\}$.
* "U is taller than at least 3 persons" $⟹$ U can be ${\text{P}}_1,{\text{P}}_2,$ or ${\text{P}}_3$.
* "Y is taller than U and X is shorter than U" $⟹\text{Y}>\text{U}>\text{X}$.
* Since $\text{Y}>\text{U}$, U cannot be ${\text{P}}_1$. Thus U is either ${\text{P}}_2$ or ${\text{P}}_3$.
* Let's check if $\text{U}={\text{P}}_2$: Then $\text{Y}={\text{P}}_1$. Since $\text{X}$ is shorter than $\text{U}$, X must be in $\{{\text{P}}_3,{\text{P}}_4\}$. We are given "X is not shorter than W", meaning $\text{X}\ge \text{W}$.
* The remaining slots for W, X, and Z are ${\text{P}}_3,{\text{P}}_4,$ and ${\text{P}}_6$.
* Since $\text{X}\ge \text{W}$ and both must be taller than V (${\text{P}}_5$), X must be ${\text{P}}_3$ and W must be ${\text{P}}_4$. Consequently, Z must be ${\text{P}}_6$.
* Let's verify the ordering: $\text{Y}({\text{P}}_1)>\text{U}({\text{P}}_2)>\text{X}({\text{P}}_3)>\text{W}({\text{P}}_4)>\text{V}({\text{P}}_5)>\text{Z}({\text{P}}_6)$.
* Let's check all conditions:
* U (${\text{P}}_2$) is taller than 4 persons (at least 3) — True.
* W (${\text{P}}_4$) is taller than V (${\text{P}}_5$) — True.
* V is taller than exactly 1 person (Z) — True.
* X (${\text{P}}_3$) is not shorter than W (${\text{P}}_4$) — True.
* Y (${\text{P}}_1$) is taller than U (${\text{P}}_2$) and X (${\text{P}}_3$) is shorter than U (${\text{P}}_2$) — True.

* The 4th tallest person (${\text{P}}_4$) is W.