Just curious since it looks like most of the big instances have already upgraded. Not sure what all is fixed in 0.17.4 but I assume any fixes are good.
It has some database optimizations and changes to the "hot" ranking algorithm.
The release notes (https://github.com/LemmyNet/lemmy/releases/tag/0.17.4) say that the database migration can take a while, so there might be downtime involved in the deploy. But it also says it fixes security issues, so that should probably be prioritized.