The solo that is one of the few solo parts for the 4. horn has both low and high notes. It begins with down beat and the solo for the 4. Horn enters very clearly. In this solo it is very critical for horn players to manage the air correctly. Because the sound should be softer and marcato, the pressure in diaphragm must be powerful during the passage. Because it has very low tones and must have a big sound, player should get deep and large breathes.
The beginning tone B flat (sounding) must be very sure and not so heavy to give the down beat feeling. For two bars the tone quality should not change: Every not should be equal and straight. After an eight note rest the pressure in diaphragm should be more intense. The tricky passage is the next one: Musically, the phrase should not be interrupted, but if you really need, you may think to inhale two bars later, before the quarter note G that is connected to the E flat. When the notes go down, it should be more forte. The next part is a very bare one, because horn gets alone and goes on to the high register. It starts like downbeat. The sound should be more dramatic and dark. The frequently done mistake is to get the tempo slower. This is both musically and technically difficult. The solo is tiring and risky. My opinion is that it is better to hold the tempo to some extent. Before beginning this bare part, player should inhale deeply, but not too much, and be economical. To achieve the right performance, player should think that the focus point is the phrase. Although it is a long sentence, it has one tone as a focus. This is F flat which comes after crescendo-decrescendo. Musically and technically one should think horizontally straight and decisive. Only one breath should be enough for this sentence. Performer inhales and holds it and exhales stable and not much. Right after the retardation, on C flat there is still no decrescendo, because the diminuendo is later than the first C flat. Than holding B flat for four beats is a preparation for the most important phrase in the solo. Inhale after B flat and start very gentle and decisively the solo. The air flow should always be fluent and it should react before articulations. It helps the player for more safety. On the third beat in this bar many musicians do ritardando: It should be though just a little, otherwise you may slow down the orchestra, which is not good. Performer may inhale twice in this phrase; first before the sixteenth notes and secondly after that bar. The rest of the solo is a link to the orchestra again but the crescendo on F flat notes is important. The music shifts from one instrument to many others, so that it gets bigger and bigger, is important.
Reference: http://imslp.org/wiki/Special:ReverseLookup/36803 Page 18