multiple image without space

Question

I am trying to show multiple images without a gap like this.

\begin{frame}
\begin{figure}
\centering
\begin{subfigure}{.5\linewidth}
  \centering
  \includegraphics[width=\linewidth]{burst_sapiro/REG_001.png}
  \caption{A subfigure}
  \label{fig:sub1}
\end{subfigure}%
\begin{subfigure}{.5\linewidth}
  \centering
  \includegraphics[width=\linewidth]{burst_sapiro/REG_002.png}
  \caption{A subfigure}
  \label{fig:sub2}
\end{subfigure}
\caption{A figure with two subfigures}
\label{fig:test}
\end{figure}

I have tried the above code, it gives me a satisfactory result, there is no gap between images.

But when I go to more images, thing goes wrong:

\begin{figure}
    % \captionsetup[subfigure]{labelformat=empty}
  \centering
  \begin{subfigure}{.25\linewidth}
    \centering
    \includegraphics[width=\linewidth]{burst_sapiro/REG_001.png}
    % \caption{Coffee.}
  \end{subfigure}
  \begin{subfigure}{.25\linewidth}
    \centering
    \includegraphics[width=\linewidth]{burst_sapiro/REG_002.png}
    % \caption{More coffee.}
  \end{subfigure}
  \begin{subfigure}{.25\linewidth}
    \centering
    \includegraphics[width=\linewidth]{burst_sapiro/REG_003.png}
  \end{subfigure}
   \begin{subfigure}{.25\linewidth}
     \centering
    \includegraphics[width=\linewidth]{burst_sapiro/REG_004.png}
  \end{subfigure}
  \end{figure}

As you can see, I do not understand why the four images are broken into two lines, and there are gaps between images. May anyone kindly give me some instruction on how to arrange images in one line without gap.