4. Use square tiles to cover a piece of land without overlapping or leaving gaps. If tiles with a side length of $x$ cm are used, exactly $n$ tiles are needed; if tiles with a side length of $y$ cm are used, exactly 124 more tiles are needed. Given that $x, y, n$ are integers, and $x, y$ are coprime. Question: How many square meters is this piece of land?
From the problem, we know
$$
n x^{2}=(n+124) y^{2} \text {. }
$$

Given that $x$ and $y$ are coprime, we have
$$
\begin{array}{l}
n+124=k x^{2}, n=k y^{2}\left(k \in \mathbf{N}_{+}\right) \\
\Rightarrow 124=k(x-y)(x+y) \\
\Rightarrow\left\{\begin{array} { l } 
{ x + y = 3 1 , } \\
{ x - y = 1 }
\end{array} \text { or } \left\{\begin{array}{l}
x+y=2 \times 31, \\
x-y=2
\end{array}\right.\right. \\
\Rightarrow x=16, y=15 \\
\Rightarrow n=900 .
\end{array}
$$

Thus, the area of the land is
$n x^{2}=900 \times 16^{2}=230400$ (square centimeters) $=23.04$ (square meters).