How do I make 2nd to 4th level categories to be indexed on google?
https://yourwebsite.com/electronics/laptops-computers/laptop/

I have submitted them but they are not indexed ,

I am using different meta tittles and different meta description for all categories including child categories to prevent duplicate

If the do not have noindex tags, google will index them when needed.
Google may not index all category pages as they all looks similar/same

I changed noindex to index in search.php only and some categories still not indexed automatically.

Is there any other php file I have to change the noindex to index for categories?

you don't have to change anything, the only thing you have to do is fill the categories with content, post an article per category, then they will also be indexed by google

Posting categories for each subcategories seems to be tiresome as I have a lot of sub categories

Also blogs prevent categories from ranking. This is because they take over the keywords which categories should have ranked for.

as long as the categories and subcategories contain no content, it will not be indexed. anyway you don't have to change anything.

if you want to get good results from this script, you should use it as is without making any changes.

in conclusion. with too many subcategories your visitors will have difficulties to find something in the mobile version.

Yes, no content = no index.
Maybe it make sense to start with few categories and expand them as you grow