Whether 4chan is a good data source or not depends on what you intend to use the AI for. If you want to have it interact with users on a web forum or similar context then using 4chan data would likely be very useful indeed.
Bear in mind that as long as it's properly labelled then "bad" data is still useful as an example of bad data. A common example is with image AIs, where people can give negative prompts like "ugly" and "blurry" to tell the AI to make images that are not like that.