Category Military: Under. What circumstances might a person enlist in Marines, but switch to Airforce enlisted and retire?

Your last EIGHT years have to be in the service and in the same condition of service. If you do four years in the Marines then 16 in the Air Force, you retire in the Air Force. There is a danger here. Lets say you do 4 years in the Marines and then come into the USAF as a reserve officer. Now you do 12 years as that and then get a regular commission as an officer. You have already done 16 years but you need 8 as the regular officer to retire so you need a total of 24 years to do it. That is why you need to ensure your last eight years have no major changes in service or condition of service. If you are a reservist on extended active duty then stay that way.

You serve your full 4-6 year enlistment in the Marines, and then once that is done you get out and then you can try to enlist in the Air Force. Right now, I don't think the Air Force is accepting prior service enlistees, but you can always ask.

That type of US military dual service used to be common in the past, say 20 years ago. Now it's difficult as the services don't want to enlist prior service people, even of their own service. The exception is the US Army, who will sometimes enlist prior service personnel of other services.

enlist in the marines for 4 years

then after leaving the marines, attempt to enlist in the air force as prior service and then serve the next 16 years in the air force