Because HD isn't quite high enough, we're taking the Saturn to new extremes. Revel in the glory of your favourite 3D titles rendered at a mind-boggling 3840x2160 - for comparison, one of the most used resolutions on the Saturn was 352x224).
Now, it isn't all roses. A large portion of the Saturn's Library was made of 2D games, which benefit nothing from an increase in resolution. Of the 3D games available, a large portion has moderate to serious problems running in YabaSanshiro - and many don't run at all - and of those that do, some don't benefit that much. Polygon jittering is a recurring issue, or 2D planes like the ground that don't scale that well and seem out of place.
But then there are models that just look gorgeous. Look at the player characters in Anarchy in the Nippon, Fighters Megamix or Virtual-On, and tell me they don't look straight out of a Dreamcast game.
As always, try it at your own risk. You might surprise yourself ;)
In case you missed the previous entries, check them out:
Part 1: Shaders in Retroarch
Part 2: Saturn Games in HD
Part 3: Widescreen Hacks