Step 1: Fix the far-left anchor.
We are told $W$ is far left, so position $1$ is $W$.
Step 2: Seat V next to W.
Since W is immediately left of V, position $2$ must be $V$. So far the line reads $W, V$ then four open seats.
Step 3: Keep Y between V and Z.
Y must sit between V and Z, so the block runs $V, Y, Z$ or $V, Z, Y$ starting from position $2$.
Step 4: Respect that X is never at an end.
X cannot take position $1$ or position $6$, so X must land somewhere in the middle four seats.
Step 5: Use U is right of Z.
U has to appear somewhere to the right of Z, which pins U toward the right side and leaves the $4$th seat to be filled by either X or Y depending on which valid ordering we pick.
Step 6: Read the 4th position.
Checking the arrangements that satisfy every clue, the $4$th seat from the left can hold either $X$ or $Y$.
\[ \boxed{X \text{ or } Y} \]