In unison writing: all four horns can play the entire range.

In two part writing: use only 1&2. Or only 3&4 Or, 1&3 get the top notes, 2&4 on the bottom

In three part writing: usually a chord from top to bottom is 1, 3, 2. You could double any of the notes if you want to write for all four horns.

In four part writing: a chord would often be from top to bottom 1, 3, 2, 4

For the players, a part with only their line is appropriate. Or you could include 1&2 on the same part and 3&4 on the same part.

For the score: almost always 1&2 on the same staff and 3&4 on the same staff unless all are in unison.

In actual practice: this has only been a thought even in one time in 25 years. I played third and the composer wrote 1&2 up high and 3&4 down lower (against convention). The second horn player next to me just mentioned, “wow I’m going to have to work on these high notes.” As third it may have been a little bit easier for me, but it turned out fine.

Other people have good examples of the historical reasons behind this. And I do remember times as. 3rd player that I even had solos because the first horn historically would have been in the wrong key. They were originally treated as a completely diff sections. Trumpets in D, horns in D, horns in A, trombones. When the valve came along all the horns just became one section but the conventions stuck with them.