Add name of instrument

You will need to press I to open the instruments dialog. You will notice there are two trombones with two staves each. Unfortunately there is no way to separate staves from an existing instrument. You can then add a trombone under each of the existing trombones that have two staves. Add a trombone and move it up with the arrow in the middle of the dialog. Add another and it will be where you want it. Click OK. You will then need to copy every thing from the second staff the the third staff (one you just added) then copy everything from the fifth staff to the sixth staff (the other one you just added). Next open the Instruments dialog and click the second staff on the first trombone that has two staves and click delete. The click the second staff on the third trombone listed and click delete. This will leave you with 4 independent trombones you can rename as you like. This will also make it easier to have independent dynamic markings on each staff if desired.