I heard about it yesterday.
In theory, their simple, old school site design could have been robust, approaching unhackable even. Unfortunately, it looks like they wrote the bulk of the site 15 or 20 years ago and didn't bother to keep their codebase and programming practices in line with the current. When faced with deprecated methods requiring significant effort to eliminate, they instead chose to run out of date supporting software such as a very old version of PHP. Additionally, shortcuts were taken, such as the use of MD5 as a password hash.
On the upside, 4chan has functionally stayed the same for the past 20 years, which is no doubt a good thing in the eyes of it's users. The maintainers had very little work to do, which made their jobs easy. On the downside, 4chan got hacked and jannies were doxed, and if 4chan wants to come back for real, someone has a lot of work ahead of them.
This, by the way, is why running old, unpatched, Internet facing software is a bad idea. This could happen to any site using software with known vulnerabilities, such as the old version of XenForo used by Kiwi Farms, or (possibly) the version PHP it runs on if it depends on some deprecated function like 4chan did. New security holes are found all the time, as well, which is a big part of why software updates come on the regular.