Batch Merge 4 Images onto a 4x6

JJMack's scripts are great for this task.

If you did have an action to layout 4 image layers into a final 4up layout, then I could supply a script that would layer images in sets of 4 and then call the action, save and then move onto the next four images etc.

Similar to the following script, however changes would be required:

Beside @JJMack script which is easiest way you have one more option to try: data sets. It needs some preparations to rename files and to create data sets, by the way. 

Here are my tutorials which explains how data sets works https://www.designeasy.co/2014/08/easy-way-to-automate-photoshop-with.html You have links at the bottom of each tutorial.

If you image all have a 2:3 portrait aspect ratio you could create a 4" x 6" 300 PP Photo collage template with 4  2" x 3" images and batch populate the template with one of the Batching scripts in my Free hotoshop Photo Collage and Mockuo Toolkit 

If your image have different aspect ratio the script will resize your image to fill the 2" x 3" image areas and mask off the excess image content. Your images will look the were centered crop to a 2:3 aspect ratio.