Pre-suppose I have pictures numbered 1 to 400. I need to simultaneously collate every 4 pics in 1, so that I end up with 100 collages. Picture 1-4, 5-8, 9-12 ... will be combined. Indubitably it's inefficient to run IrfanView's "Create panorama images" 100 (400/4) separate times!
In your answer, you must stipulate whether the software is free...I prefer freedom, but if no free software, then I can pay. I use Windows 10. I know no programming or computer science.