How many basses are said stock parts written for. If 4, it will sound the best played as it is written. If 3, it will sound OK but not great. If 5, then it will sound much better modified for 4 then for 3.
4 also opens up more possibilities phrase wise in writing for cadences and warm ups, but again, 3 can be OK.
Personally I'd say 4, but it's really a call based on the exact demands of the situation.
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WHRHS Marching Band, 2007-2011
Purdue University ALL AMERICAN Marching Band, 2011-????