Would make sense that Grade 4 is the lowest level of training (as opposed to rank) held by Recruits, counting up Grades 3/2 and 1, for either (Able) Crewman/Crewman 1st Class, Petty Officer and Chief?
All of this is great gee-wiz stuff, but it's also pretty mundane and would bore most readers to death. It might be mentioned in passing if it somehow affects the story line. The computer won't let Crewman Jones tack on Petty Officer Third Class because Ensign Smith dropped the ball on getting the training records updated with the proper job-skill rating. Smith is fired from his cushy job and later tries to pull a Ben Finney move.
This chart may be of some interest to you: https://www.defense.gov/About-DoD/Insignias/Enlisted
AIR FORCE
Airman Basic (E-1) - 13,693 (4.7%)
Airman (E-2) - 12,712 (4.4%)
Airman First Class (E-3) - 53,094 (18.3%)
Senior Airman (E-4) - 53,223 (18.4%)
Staff Sergeant (E-5) - 72,388 (25.0%)
Technical Sergeant (E-6) - 43,992 (15.2%)
Master Sergeant (E-7) - 31,061 (10.8%)
Senior Master Sergeant (E-8) - 5,681 (2.0%)
Chief Master Sergeant (E-9) - 2,876 (1.0%)
NAVY
Seaman Recruit (E-1) - 15,421 (5.1%)
Seaman Apprentice (E-2) - 16,549 (5.5%)
Seaman (E-3) - 51,001 (16.9%)
Petty Officer Third Class (E-4) - 58,644 (19.4%)
Petty Officer Second Class (E-5) - 2,552 (24.0%)
Petty Officer First Class (E-6) - 54,019 (17.9%)
Chief Petty Officer (E-7) - 23,359 (7.7%)
Senior Chief Petty Officer (E-8) - 7,339 (2.4%)
Master Chief Petty Officer (E-9) - 2,936 (0.9%)
MARINES
Private (E-1) - 9,671 (6.3%)
Private First Class (E-2) - 20,625 (13.4%)
Lance Corporal (E-3) - 43,141 (30.0%)
Corporal (E-4) - 29,578 (19.2%)
Sergeant (E-5) - 23,019 (15.0%
Staff Sergeant (E-6) - 14,794 (9.6%)
Gunnery Sergeant (E-7) - 8,801 (5.7%)
Master Sergeant/First Sergeant (E-8) - 3,306 (2.1%)
Master Gunnery Sergeant/Sergeant Major (E-9) - 1,413 (0.9%)
ARMY
Private (E-1) - 20,284 (5.1%)
Private (E-2) - 3,334 (9.1%)
Private First Class (E-3) - 56,757 (14.1%)
Specialist/Corporal (E-4) - 107,634 (26.8%)
Sergeant (E-5) - 73,034 (18.2%)
Staff Sergeant (E-6) - 56,664 (14.1%)
Sergeant First Class (E-7) - 36,725 (9.2%)
Master Sergeant/First Sergeant (E-8) - 10,541 (2.6%)
Sergeant Major (E-9) - 3,165 (0.8%)
USAF
O-1 9803 14.1%
O-2 8208 11.8%
O-3 21706 31.2%
O-4 15361 22.1%
O-5 10362 14.9%
O-6 3753 5.4%
FLAG 273 0.4%
NAVY
O-1 7824 15.0%
O-2 6873 13.2%
O-3 16431 31.4%
O-4 10483 20.1%
O-5 7028 13.5%
O-6 3390 6.5%
FLAG 220 0.4%
USMC
O-1 2650 16.2%
O-2 2805 17.1%
O-3 5258 32.1%
O-4 3202 19.5%
O-5 1770 10.8%
O-6 622 3.8%
FLAG 81 0.5%
ARMY
O-1 6206 9.6%
O-2 8782 13.6%
O-3 22768 35.3%
O-4 14248 22.1%
O-5 8633 13.4%
O-6 3564 5.5%
FLAG 301 0.5%
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