Why does including proj4js.js cause a 404 error re: merc.js can’t be found, even though merc.js is not mentioned anywhere in proj4js.js?

I am working on a page with a lot of code including an interactive map. (The huge majority of the code was written prior to my involvement.) Every time the page is opened, there is a 404 error in the console because projCode/merc.js cannot be found. I want to get rid of this error, but I have no idea why it is coming up at all because I cannot find any code that references a file called merc.js – except for the comments in proj4js-combined.js, which imply that merc.js used to be a separate file but its contents are now included in the ‘combined’ file.

The HTML file for the page includes the following:

<script src="~/Scripts/Maps/proj4js-combined.js"></script>
<script src="~/Scripts/Maps/proj4js-compressed.js"></script>
<script src="~/Scripts/Maps/proj4js.js"></script>
<script src="~/Scripts/Maps/proj4.js"></script>

I notice that commenting out the proj4js.js line makes the error go away, but I have no idea why. As I say, nothing in proj4js.js references “merc.js”.

Is the solution just to not include proj4js.js? Or will that cause a loss of functionality, and there is some other problem here?