I've looked at upgrade notes - but I'm kind of looking for a "if you're used to 3.x here's what you need to know" type of guide
hmm, upgrade docs are pretty good, with old code and new code. There is also an upgrader tool that gets you started, https://github.com/silverstripe/silverstripe-upgrader
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if there isn't db details in config.php, and there isn't a __ss_environment.php file, is there anywhere else a SS install will look for those details?
SS4 has .env or Environment Variables, but afaik that's it
i'm looking at somebody's site, can't really see how it's connecting. no _config.php or _ss_environment.php
_ss_environment.php can be above webroot, afaik it scans all dirs up