For day 4, plan your cemetery trip in advance if there are specific sites you want to see. I worked there for 2.5 years and you can do a lot of walking and not see a whole bunch.

Changing of the guard is on the hour (or every 30m from 1 Apr to 30 Sep) and best to do at this time of year in the mid-afternoon. Ed: since your flight is in the afternoon, mid or late morning is equally good.

Get the ANC Explorer app on your phone or visit the website - https://ancexplorer.army.mil/publicwmv/#/arlington-national/.

You can walk one of the pre-built tours, customize them, or walk your own path. The south and east of the cemetery are newer sections, whereas the north and west are older (as a general rule of thumb). If I were to recommend something so that you see some decently cool stuff there in a compact time, you can check out the area between the confederate section in section 16, east to Memorial Amphitheater where the Mast of the Maine and Audie Murphy's grave are before going in to see the changing of the guard. Then, head north along Crook Walk and just east to the JFK and RFK, then back west past Tanner Amphitheater and the Civil War Unknown to section 1 in the NW near the gate to Fort Myer.

If you get a little more time, I'd check out General Hill in section 30 by Taft's grave (just NW of the Welcome Center). And, while you're on Crook's Walk, you can go visit the Arlington House, if it's open. I am not sure, since it's run by the National Park Service rather than the Army, and I've never been. Or you can check out the Notable Graves and devise your own points of interest.

If all that walking (and the hills) aren't appealing, there are tram tours at cost, but they come with a guided tour and a few stops.

Also, obligatory comment that I no longer work there because the organization's leadership is incredibly toxic and tries to cover up anything that could lead to bad publicity. But enjoy your visit.