Hi Anssi!
Yes, dividing chords to three or four players automatically is possible. As you have already assumed, you will need VI PRO for that. The normal "VI" doesn't have the "Voice" feature for auto-divisi. However, it's possible to notate every voice on a seperate staff.
Of course you can load all players more than once. Then you can have as many horns, as you want. For fat unison horn lines, I would recommend our Epic Horns. We recorded eight horns for that product. These are available in the Brass II collection or as a single download instrument.
Here are the recorded ranges of the Dimension Brass:
Dimension Horns: B1-D5
Dimension Trumpets: E3-C6
Dimension Trombones: E2-C5
Dimension Low Brass: B1-C4 (Bass Trombone and Tuba sounding an octave lower)
Best regards,
Andi
Vienna Symphonic Library