To find the height of Plant B after 4 months using the given equation \( h = 8 + t \), we can substitute \( t = 4 \) into the equation:
\[ h = 8 + 4 \]
Now, we can perform the calculation:
\[ h = 8 + 4 = 12 \]
So, the height of Plant B after 4 months is 12 cm.