If you have little to no experience in php you may be dead in the water on the natsort ; (
I do not have a working example to test the code on so I can't test it on my machine, but you might be able to get away with adding the natsort to the field Caption in the sort function. As the $fields variable builds caption as an array. So you could start there. If it doesn't work there you could try to use the natsort on the $fields variable where and when it appears in the sort function, but that is going to change all 4 fields. So you might wanna make a separate field variable for caption like
$caption and then build on the sort function to just use that variable as well.