To find the value of \( t \) when the heights of Plant A and Plant B are equal, we can set the two equations equal to each other:
\[ 4 + 2t = 8 + t \]
Now, let’s solve for \( t \):
- Start by subtracting \( t \) from both sides:
\[ 4 + 2t - t = 8 \]
This simplifies to:
\[ 4 + t = 8 \]
- Next, we subtract 4 from both sides:
\[ t = 8 - 4 \]
This gives us:
\[ t = 4 \]
Thus, the value of \( t \) when the heights of Plant A and Plant B are equal is \( \boxed{4} \) months.