Upgrading 3.0.3.3 to 3.0.3.8 or version 4

All, Currently I am using OpenCart 3.0.3.3 with I think the default theme. I am planning to upgrade to 3.0.3.8. I have a number of questions.

- Is 3.0.3.8 secure enough or is it always better to use version 4?
- I have found this page to upgrade the opencast version: https://docs.opencart.com/upgrading/. If I read it properly the only thing I need to do is to copy the //upload/admin from my local site to my online site. Then select the xxx.com/install page. Is this correct? The page is not very clear on that.
- Is there another tutuorial to do this easily? (without any other software)
- Is there a difference in upgrading from 3.0.3.3 to 3.0.3.8 or if I upgrade to the version 4 instance? Is it more difficult?
- For my OpenCart installation I have changed some pages by changing css or php files. Will this be undone by the upgrade?

Thanks!

OC v3.0.3.9 supports full PHP 8 and 8.1. Currently on the hold for PHP 8.2: https://www.github.com/opencart/opencart-3