Most common 4 speed transmission leaks are attributed to the following areas.
I.D. or O.D. of the large main drive gear seal
Between the shoulder of the main drive gear and the backside of the main drive gear spacer. This must maintain a tight metal to metal contact. This is the most commonly overlooked leak. A notch gets worn into the spacer where it contacts the main drive gear and bearing, where #2 points to. The notch often looks like it was machined in and looks like it belongs there, it does not. This leak occurs after the sprocket nut (#7) loses clamp load for any amount of time. The spacer has to be replaced if worn.
I.D. or O.D. of the small main drive gear seal. Plus, note if the mainshaft has a groove from lip of oil seal.
I.D. of main drive gear bushing. Worn bushing allows the main drive gear to rock on the mainshaft causing small main drive gear seal leaks.
Main drive gear bearing, noted for location only. Not a cause of leaks.
Where the oil leaks to in #2 and why it looks like it comes from #3.
Sprocket nut, noted for location only. Not a cause of leaks.
You can clearly see the step to which I refer in number 2 above. There is not suppose to be a step in the part, the surface is suppose to be flat. This is caused by the clamp load being lost. This is often overlooked because it looks like it is machined in, and suppose to be there but it is not.
Understanding this has allowed me to resolve transmission leaks that others could not.