A little schedule surfing reveals:
AC4000 series flights are North American codeshares with OO (Skywest)
AC5000 series flights are North American codeshares with UA
AC8000 series flights are Air Canada Jazz
AC9000 series flights are European codeshares with a variety of carriers
AC9100 series flights are codeshares with LH (both European and Trans-Atlantic)
AC9300 series flights are codeshares with SK (Scandanavian)
AC9500 series flights are codeshares with NZ (Air New Zealand)
AC9600 series flights are codeshares with OS (Austrian)
AC9700 series flights are codeshares with MX (Mexicana)