2x2 unequal image placement, Subcaption. First two image are stacked horizontally. second two vertically, why?

Question

I have 4 images of various sizes (attached herewith). I want to place them in a 2 x 2 grid. Their widths should be the same, roughly 0.5/textwidth. I have done the following:

'''

\documentclass[12pt,onehalfspacing,headsepline,oneside,openright,a4paper]{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}
    \centering
    \begin{subfigure}[t]{0.5\textwidth}
    \centering
        \raisebox{-\height}{\includegraphics[width=.8\linewidth]{images/image1.jpg}}
        \caption{Step 1: Creating a new Project}
        \vspace{1.7cm}
        \label{fig:newproj}
    \end{subfigure}%
    \begin{subfigure}[t]{0.5\textwidth}
    \centering
    \raisebox{-\height}{\includegraphics[width=.8\linewidth]{images/image2.jpg}}
        \caption{Step 3: Creating a new Model}
        \label{fig:newmod}
    \end{subfigure}
    \newline
    \begin{subfigure}{0.5\textwidth}
    \centering
        \includegraphics[width=0.8\linewidth]{images/image3.jpg}
        %\vspace{0.7cm}
        \caption{Step 5: Choosing model subsystem structure}
        \label{fig:smsc_structure}
    \end{subfigure}
    \begin{subfigure}{0.5\textwidth}
    \centering
        \includegraphics[width=0.8\linewidth]{images/image4.jpg}
        %\vspace{0.7cm}
        \caption{Step 7: Editing new Model}
        \label{fig:smsc_structure}
    \end{subfigure}
    \caption{How to make a model in OPAL-RT}
    \label{fig:scratch}
\end{figure}
\end{document}

'''

The output looks like this:

But what I want is :

To explain in words, I want the top margin and bottom margins of the figures to be flush with each other horizontally. I should be able to do the fine-tuning by using space to add space between sub-figures and captions.

Any insights about what I am doing wrong would be greatly appreciated.

Note: I am new to this forum and I am trying my best to adhere to the guidelines. Kindly give me a chance to rectify any mistakes made before downvoting.

UPDATE: With the suggestion from @David Carlisle. I have added '%' a comment after the \end{subfigure}. And Adding [t], makes the second row flush to the bottom. This is how the result looks like:

However, as pointed out, the numbering is now jumbled up (I need c and b to be interchanged). How do I fix that? Updated code is as follows:

'''

\documentclass[12pt,onehalfspacing,headsepline,oneside,openright,a4paper]{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}
    \centering
    \begin{subfigure}[t]{0.5\textwidth}
    \centering
        \raisebox{-\height}{\includegraphics[width=.8\linewidth]{images/image1.jpg}}
        \caption{Step 1: Creating a new Project}
        %\vspace{1.7cm}
        \label{fig:newproj}
    \end{subfigure}%
    \begin{subfigure}[t]{0.5\textwidth}
    \centering
        \raisebox{-\height}{\includegraphics[width=.8\linewidth]{images/image3.jpg}}
    %\includegraphics[width=.8\linewidth]{images/image2.jpg}
        \caption{Step 5: Creating a new Model}
        \label{fig:newmod}
    \end{subfigure}
    \newline
    \begin{subfigure}[t]{0.5\textwidth}
    \centering
        \includegraphics[width=0.8\linewidth]{images/image2.jpg}
        %\vspace{0.7cm}
        \caption{Step 3: Choosing model subsystem structure}
        \label{fig:smsc_structure}
    \end{subfigure}%
    \begin{subfigure}[t]{0.5\textwidth}
    \centering
        \includegraphics[width=0.8\linewidth]{images/image4.jpg}
        %\vspace{0.7cm}
        \caption{Step 7: Editing new Model}
        \label{fig:smsc_structure}
    \end{subfigure}
    \caption{How to make a model in OPAL-RT}
    \label{fig:scratch}
\end{figure}
\end{document}

'''

Answer 1

A subfigure is positioned by the same logic as a letter you have

WX
\newline
Y Z

where each of the "letters" is half of the text width.

So on the first line you get two just fitting but on the second line the word-space between them would make them not fit so you get an automatic line break at the space.

In your case the word space comes from the end of line after \end{subfigure} in the case that it is not commented out.

Answer 2

I am posting my answer from inputs received from David Carlisle and How to vertically label the subfigures?. I shall not accept my answer as right unless the community agrees. If not I shall choose David's answer as the right one, even though it did not exactly provide a solution.

'''

\documentclass[12pt,onehalfspacing,headsepline,oneside,openright,a4paper]{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}
\begin{subfigure}[t]{0.5\textwidth}
%   \includegraphics[width=0.9\linewidth]{images/image1.jpg}
  \raisebox{-\height}{\includegraphics[width=.9\linewidth]{images/image1.jpg}}
  \vspace{0.25cm}
  \caption{First subfigure} \label{fig:1a}
  \vspace{0.25cm}
  \par\medskip % if more vertical separation needed, use \bigskip
  \includegraphics[width=0.9\linewidth]{images/image2.jpg}
  \caption{Second subfigure} \label{fig:1b}
\end{subfigure}
\hspace*{\fill}
\begin{subfigure}[t]{0.5\textwidth}
    \raisebox{-\height}{\includegraphics[width=.9\linewidth]{images/image3.jpg}}
  %\includegraphics[width=0.9\linewidth]{images/image3.jpg}
  \caption{Third subfigure} \label{fig:1c}
  \par\medskip % if more vertical separation needed, use \bigskip
  \includegraphics[width=0.9\linewidth]{images/image4.jpg}
  \caption{Fourth subfigure} \label{fig:1d}
\end{subfigure}
\caption{This is a figure with six subfigures}\label{fig:1}
\end{figure}
\end{document}

''' The outcome is as expected