If the amp is set to 4Ω, then that's the MINIMUM impedance that the output stage expects to see.
Anything over that (e.g. an 8Ω cab) is fine; you'll get a reduced output from the amp, so it won't be as loud (so you'll have to boost the master volume control to compensate), but a 1000W amp should still have tons of power, even into an 8Ω cab.
No smoke or flames though.