increase number of walker to 4

./scripts/assembly_debugging/SAMN02402663-Mtuberculosis-spate-4x8.sh

Before (1 actor / worker):
3m4.194s

After (4 actors / worker):
3m4.283s

Link: https://github.com/GeneAssembly/biosal/issues/660
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>