The \\ in the diagnostic output is not the same thing as what is actually being used for the execution; it’s an artifact of how the debug output is captured and displayed (because sequences that start with \ are special to programming languages). So, that is unlikely to be your problem.
In regards to Python builds, the change that happened between ST3 and ST4 is that the build system uses py instead of python on Windows because the most common problem people had with running Python programs from Sublime were caused by them installing Python without adding it to the path. However, Python has a py launcher which can find the correct Python install even if it’s not on the path, and py is installed into the Windows system folder where it’s always available.
This causes issues for people that never told their Python installer to install the py launcher (or if you are using a Python variant that does not include one), because now the build system is trying to launch a thing that does not exist.
Presuming that your system actually has Python installed, more information on this and how to resolve it can be found in the video below. If you don’t have Python installed, then you should do that first (but based on your screenshot, you likely do since there seems to be something Python related on your PATH).
