Can someone please comment on 4th gen batteries in general when it comes to DIY maintenance projects and clearing codes, possibly introducing unwanted codes by messing with it etc.

I've read some instances around where unhooking the battery actually made things worse or introduced new codes..

I don't know that I can specifically speak to the 4Runner or its batteries, but having gone through this for years with my RX300, I am fully aware that disconnecting the battery will reset the vehicle computer so that it has to relearn your 'driving style' so to speak. This doesn't usually take too long, maybe 15 to 30 minutes, mixing city and highway (going at least once above 55 mph).

I've never heard of something getting worse from doing this (such as harming the vehicle), though of course you can lose presets and if you floor the gas shortly thereafter (treat it like a race car), well you're mph is going to suffer.

I'd be curious to know what you read regarding disconnecting the battery and making things worse as I might be able to shed some light on it.