I’ll be selling my 304 since I bought a 604, so I wanted to post this before I forget, to help out others who bought one and might be under-utilizing their presets. They are actually pretty useful.
Basically Yamaha gave them all names for styles they would be good for, but the thing is, some are more generally useful, so here is what they actually are. This is not in the manual. From top to bottom on the switch:
Slap - This is actually a mid scoop, much like the “contour” button on Fender amps. This is almost always useful, depending on style.
Pick - slight mid scoop with a treble boost. Actually useful too, might want to add a little bass.
Flat - it does what the package says, nothing. Always useful.
Finger - high end is EQed away, which is nice for killing finger noises on strings. Not super useful compared to Solo in my experience.
Solo - same as Finger, but with a low mid boost. Useful because it’s the only real mid boost preset, along with killing the high end.
Helpful chart for reference: