@uqaz+1fFnNBsK
Military band is the place where you can see people starting at E6 - which is past where your self identified glass ceiling is. Anyone can start in or transition to the band if they have the skills (merit).
https://www.rallypoint.com/answers/is-it-possible-to-get-promoted-this-quickly-in-the-air-force-is-this-typical-for-band-members
https://www.quora.com/How-do-members-of-the-US-military-view-the-US-Marine-Band-given-that-bandmembers-do-not-attend-boot-camp-are-never-deployed-into-combat-and-yet-start-their-careers-at-the-E-6-rank
Then for the enlisted to officer programs - no promotion from enlisted to enlisted will ever come with as many perks as enlisted to officer does. You mentioned starting straight in OCS, which is not "gaining rank" but instead it is starting with rank. You can also avoid bu-t kissing by going through these enlisted to officer programs if you have the line scores in your ASVAB (merit).
Not all MOS's are created equal when it comes to being a Warrant Officer. I was surrounded by WO's and CWO's where I was stationed. They were all hardcore and would tell anyone, regardless of rank, to pound sand. I never got a bu-t kissing vibe from a WO.
Then for your original question of:
"If I’m wrong, explain to me why so many soldiers wash out at E4"
This is simply math. Another way to ask this question might be to say - Why do so many individual contributors leave before becoming a manager? If you are interested in this type of phenomena you can read about Price's Law:
"Price's square root law or Price's law pertains to the relationship between the literature on a subject and the number of authors in the subject area, stating that half of the publications come from the square root of all contributors. Thus, if 100 papers are written by 25 authors, five authors will have contributed 50 papers. Price's law is related to Lotka's law and has been likened to the Matthew Principle. It can be modeled using an approximately L-shaped graph, with number of people on the Y-axis, and productivity or resources on the X-axis." - Wiki
Another way of saying this is that if you have a company of 9 people, 3 do 50% of the work. A simple example of this would be the number of times the radio plays popular names like Bach, Chopin, Mozart, or Beethoven Vs all other composers.
I have been involved in many company conversations where the executives and managers say we have a problem with "Crown Nice," which is another way of saying bu-t kissing managers. If we have a brown lipstick problem, then why are we still promoting brown lipstick? If bu-t kissing is the deciding factor for a promotion in an "all else being equal" type of scenario, then we have a serious problem.