How to arrange multiple figures as subfigures on a page

Question

I have multiple figures on my page, from previous posts here I have tried to arrange them 2 figures at a time. I wish to make all 4 figures appear as 1 with sub-labels as (a) , (b) , (c) , (d) which are all properly aligned among each other.

Code: This code has 2 figures with 2 figures within them.

\begin{figure}[t]
\centering
\begin{minipage}{.45\linewidth}
  \pgfplotsset{every axis/.append style={scale=0.8}}
  \input{figs/FIG_5.tikz}
  \captionof{figure}{First caption}
  \label{fig:LV}
\end{minipage}\hfill
\begin{minipage}{.45\linewidth}
  \pgfplotsset{every axis/.append style={scale=0.8}}
  \input{figs/Fig12.tikz}
  \captionof{figure}{Second caption}
  \label{fig:LV2}
\end{minipage}
\end{figure}
\begin{figure}[t]
    \centering
    \begin{minipage}{0.45\textwidth}
        \centering
        \pgfplotsset{every axis/.append style={scale=0.8}}
        %\graphicspath{figs/FIG_3.tikz}
         \input{figs/FIG_3.tikz} % first figure itself
        \caption{first figure}
        \label{fig:EW}
    \end{minipage}\hfill
    \begin{minipage}{0.45\textwidth}
        \centering
        \pgfplotsset{every axis/.append style={scale=0.8}}
         \input{figs/Fig11.tikz} % second figure itself
        \caption{second figure}
        \label{fig:EW2}
    \end{minipage}
\end{figure}

Code Fig 5 and Fig 3:

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\pgfplotsset{
  set layers,% using layers
  mark layer=axis tick labels,% defines the layer of the marks
  every axis/.append style={scale=1.2}
}
\begin{semilogyaxis}[
xmin=0, xmax=205,
ymin=1e-1, ymax=1e5,
xlabel={Oscilatoriness parameter ($k$)}, 
ylabel={Gain in performance ($\alpha$)},
label style={font=\bfseries\boldmath},
tick label style={font=\bfseries\boldmath},
legend pos=outer north east,
scatter/classes={ a={mark=square, violet}, a1={mark=square, red},
] 
\addplot[scatter, only marks,
scatter src=explicit symbolic]
table[meta=label] {
x     y      label
5 5.67E+00 a 
5 6.67E-01 a1
};
\end{semilogyaxis}
\end{tikzpicture}
\end{document}

Code : figure 11:

\begin{tikzpicture}
\begin{axis}[axis x line=middle,axis y line=left,xlabel=$x$,restrict y to domain =-9:9, legend pos=north west ]
\addplot[orange,samples=500,domain=0:1.5]plot (\x, {exp(\x)*sin(100*cosh(\x) r)});
\addplot[orange,samples=1000,domain=1.5:2]plot (\x, {exp(\x)*sin(100*cosh(\x) r)});
%\legend{\textcolor{orange}{$\mathrm e^x\sin\left(100\cosh\left(x\right)\right)$}}
\addplot[blue,samples=500,domain=0:1.5]plot (\x, {exp(\x)sin(10cosh(\x) r)});
\addplot[blue,samples=1000,domain=1.5:2]plot (\x, {exp(\x)sin(10cosh(\x) r)});
\legend{\textcolor{orange}{$\mathrm e^x\sin\left(100\cosh\left(x\right)\right)$}, \textcolor{blue}{$\mathrm e^x\sin\left(10\cosh\left(x\right)\right)$}}
\end{axis}
\end{tikzpicture}

figure 12:

\begin{pspicture}(0,-.5)(20,1.0)
\rput(13,0.8){%
    $\displaystyle J_n(x)=\frac{1}{\pi}\int_0^\pi\cos(x\sin t-nt)\mathrm{d}t$}
\psaxes[Dy=0.2,Dx=4]{->}(0,0)(0,-.4)(25,1)
\psset{linewidth=1pt}
\psBessel[linecolor=red]{0}{-0}{25}%
\psBessel[linecolor=blue]{1}{-0}{25}%
\psBessel[linecolor=green]{2}{-0}{25}%
\psBessel[linecolor=magenta]{3}{-0}{25}%
\end{pspicture}

Answer 1

Instead of copying all your figures I just replaced them here with some example images. Nevertheless this should suit your needs.

\documentclass{article}
\usepackage{subcaption,graphicx,pgfplots}
\begin{document}
    \begin{figure}[t]
        \centering
        \subcaptionbox{First caption.\label{fig:LV}}[.45\linewidth]{%
            \pgfplotsset{every axis/.append style={scale=0.8}}%
            %\input{figs/FIG_5.tikz}%
            \includegraphics[width=0.5\linewidth,height=5cm]{example-image-a}%
        }\hfill
        \subcaptionbox{Second caption which goes over multiple lines is also aligned properly.\label{fig:LV2}}[.45\linewidth]{
            \pgfplotsset{every axis/.append style={scale=0.8}}%
            %\input{figs/Fig12.tikz}%
            \includegraphics[width=\linewidth]{example-image-b}%
        }
        \subcaptionbox{Third caption.\label{fig:EW}}[.45\linewidth]{
            \pgfplotsset{every axis/.append style={scale=0.8}}%
            %\graphicspath{figs/FIG_3.tikz}%
            %\input{figs/FIG_3.tikz} % first figure itself
            \includegraphics[width=\linewidth]{example-image-c}%
        }\hfill
        \subcaptionbox{Fourth caption. In every good example there should be a duck!\label{fig:EW2}}[.45\linewidth]{%
            \pgfplotsset{every axis/.append style={scale=0.8}}%
            %\input{figs/Fig11.tikz} % second figure itself
            \includegraphics[width=\linewidth]{example-image-duck}%
        }
        \caption{Collection of all my cool figures.}
    \end{figure}
    References: \ref{fig:LV}, \ref{fig:LV2}, \ref{fig:EW}, \ref{fig:EW2}.
\end{document}