I would stick with an 8 ohm speaker cab. You can risk it with a 4 ohm cab, or a 16 ohm cab if you want, but you would be stressing the transformer. Perhaps the transformer can handle the additional stress, but it can't be good for extended periods of time.
The lower the ohms of the cab the less resistance it offers. So an 8ohm amp run into 4 ohms will cause the amo to put out too much power for the amp to handle and may cause the tranny to blow up.
On the other hand an 8 ohm amp run into a 16 ohm load will lower the power the amp puts out. You wont blow the transformer but may prematurely burn up your power tubes.
The output transformer's main job is to convert the impedance of the output tubes to the impedance of the speaker for maximum efficiency. When there's a mismatch, there's a reduction in efficiency. Most any tube amp can withstand a 100 percent mismatch, so an 8 ohm nominal rating can drive a 4 ohm load or a 16 without problems. One notable exception: don't try to go lower on amps with 2-ohm output.
When the impedance gets too high on a tube amp, you get the possibility of flyback--arcs that can damage the output transformer or the output tubes. The BJr and many other modern amps have anti-flyback diodes in the output circuit to short out these high-voltage pulses.
When the impedance gets too low, the tubes and output transformer run warm. But it takes a lot of current, like playing loud into a dead short, to damage either.
Fender used the stock Blues Junior chassis in the Two Tone, a big cabinet with a 10 and a 12, both 8 ohm speakers, connected in parallel, a 4 ohm load.
I drive a second 8 ohm cab all the time from my Blues Juniors. So do many of my customers.