The 4, 5, 6, and 7 keys are all assigned to the INDEX fingers with the 4 and 5 assigned to the LEFT INDEX finger and the 6 and 7 assigned to the RIGHT INDEX finger. Some people prefer to hit the 6 with the LEFT INDEX finger. Do what feels best to you, but be consistent once you decide which finger to use.
Other finger assignments:
LEFT PINKY: 1-key
LEFT RING: 2-key
LEFT MIDDLE: 3-key
RIGHT MIDDLE: 8-key
RIGHT RING: 9-key and 0-key