Unfortunately you had to learn the hard way 🙁
This is not about that version 4 sucks, actually, version 4 comes with great new feature, like the new advanced orderby function in WP_Query, it is more about an outdated website.
Downgrading will not solve your problem in the long run, it might even create more problems than solving it. For now, contact your host as ask if they can delete wordpress for you and upload the version you require. Luckily, you have made a back up before upgrading
How to avoid future problems
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Set yourself up with a local testing enviroment. There are a couple of nice ones out there. I personally use XAMPP (to which I don’t have any affiliation to) for this job. I test all updates, upgrades, codes, plugins, themes etc on my local install before making changes to a live site
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Always have debug turned on on your local install. There are also one or two excellent debugging plugins available that you can install on your local install
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Try to keep the settings and configurations on your local install the same as on your live site. There is no use having PHP 5.4 on your local install and your host uses PHP 5.2.
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Do not make use of low or average hosting plans and do not use unknown hosts. Most problems come from low quality hosts. I’ve mentioned PHP 5.2, and believe it, there are hosts that still uses 5.2, which has been dropped years ago
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Keep your code up to date, not just WordPress code, but all code in general, like PHP and HTML code in your theme/plugin. Sometimes errors are caused by simple non related PHP code. Also, keep track of wordpress functions. Functions gets depreciated over time, but this should not really be a problem if you have debug turned on
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Always fix debug errors, and listen to what they are telling you. They are there for a reason, and that is to help you