Join images using Alibaba's OSS image processing

I am using Alibaba's Object Storage Service's image processing to process my images. I need a way to join (stitch) a few images together and create a larger image.

Background: I want to scale up an image to 7680 × 4320 (8k) resolution using the OSS image processing. But every time I do that, it fails as it only allows scaling images to a maximum dimension of 4096 x 4096.

A solution that I came up with, for this problem, was this:

  1. Crop my image into 4 quarters resulting into 4 smaller images

    Can be made possible with the help of the Crop operation to make parts of the initial image and the Saveas operation to save those images.

  2. Independently scale up those 4 images to 1920 x 1080

    Can be made possible using the Resize operation to scale up those image parts.

  3. Join those scaled images together to obtain the larger image

    The documentation does not state any direct way to join images. I'm looking for a way or a workaround for the same.

How do I accomplish this 3rd step, so as to join those 4 images together to form the final 7680 × 4320 image output?