With a SS amp, you can go from 4 ohm output to 8 ohm or 16 ohm speaker cab and it’s safe. It may or may not sound great, but it’s safe.
With tube amps 1:1 ohm ratings is safe. Generally, with tube amp impedance mismatches, it is considered safer going higher to lower, 8 ohm to 4 ohm (2:1)...