Prove that y_2+y_3+2y_4 \leq 6 is valid for

X= \{ y \in \mathbb{Z}_{+}^{4}: 4y_1+5y_2+9y_3+12y_4 \leq 34 \}

I don't completely understand this because it seems that y_3 could be 6, which would be greater than 34.