4. The height of a ball thrown vertically upwards from the ground is a quadratic function of its motion time. Xiao Hong throws two balls vertically upwards, 1 second apart. Assuming the two balls are thrown from the same height above the ground, and they reach the same maximum height above the ground 1.1 seconds after being thrown. If the height of the first ball above the ground is the same as that of the second ball $t$ seconds after the first ball is thrown, then $t=$ $\qquad$
4.1.6.

As shown in Figure 7, $AB=1$.
Let $C(1.1, h)$.
Then $D(2.1, h)$.
By symmetry, we know that the x-coordinate of point
$P$ is
$$
\begin{array}{l}
1.1+\frac{2.1-1.1}{2} \\
=1.6 .
\end{array}
$$