There's no real relationship between sh404SEF and 4SEO, and 4SEO is definitely not a replacement for or successor to sh404SEF as it does not deal at all with SEF URLs.
Here is the global picture:
- sh404SEF does SEF URLs, 4SEO does nothing with URL construction and management (it has redirections and canonical, a lot)
- It means you can use 4SEO with sh404SEF for SEF URLs, but also with JoomSEF or another SEF extension, or directly with Joomla SEF URLs.
- sh404SEF has few SEO features, 4SEO is only SEO
- you can use both together on Joomla 3
- 4SEO runs on Joomla 3 and 4
- sh404SEF will only run on Joomla 3
- when Joomla 4 is stable, we will have a small Joomla 3&4 extension, called 4SEF, that will take over the URL management part of sh404SEF, and provide a transparent migration from Joomla 3 to Joomla 4 for SEF URLs and custom meta data.
So there's a bit of overlap, but 4SEO is SEO for 2021. It does away with most of the extra baggage accumulated by sh404SEF but adds SEO features lacking in sh404SEF, those that matters today:
- structured data generation
- core web vitals measurement
- sitemap with only relevant URLs
- broken links identification
- redirection engine, canonical rules
- content replacement (auto linking for instance)
- several other tools needed for SEO (analytics snippets, custom code insertion, put some pages offline or 404)
- work on the frontend
and of course 4SEO has a much better user interface.
Again, however, 4SEO does not and will not deal with SEF URLs so it cannot replace sh404SEF.
In addition, I'd like to mention that sh404SEF will be supported (on Joomla 3) for as long as Joomla 3 is relevant. Joomla 3 will be supported by the Joomla project officially for 2 years after Joomla 4 is released, but I know people will keep using it for more than that so I will support sh404SEF at least another year after that, and maybe more depending on demand.
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