Matt,
The '604 tends not to be a velocity monster, so lighter pellets should give the highest velocities, and cut the cleanest holes in the target.
RWS R10 Pistol (6.99 gr), JSB Match Green (7.33 gr.), RWS Hobby (7.11 gr.), RWS Geco (7 gr.) & RWS CO2 Target (7.14 gr.) are among those worth trying to see which your '604 likes best.
Have fun & a good Sunday
Best regards
Russ