First of all, if you have a choice (and why wouldn't you?), avoid an impedance mismatch. So my advice would be to get one more speaker, either 4 or 8 Ohms to get an impedance which you can match. Then you don't need to worry, and if your amp ever dies on you, you won't have that nagging feeling that you should have known better ...
Having said that, and if you really want to use those two speakers, then
I wouldn't use a resistor, because it's a shame to use amp power for heating your cabinet. I'd say it's probably most reasonable to connect the two speakers in series, which gives you 8+4 = 12 Ohms. Then you can either choose the 8 Ohms setting on your amp, or use the 16 Ohms setting, because almost all amps can handle a slight impedance mismatch. It depends on whether you have a tube or a solid state amp, and if you have a tube amp, it also depends on the model how sensitive it is to an impedance mismatch.
If you connect the speakers in parallel you'll get a resistance of 8*4/(8+4) = 2.67 Ohms, which is quite small, and which I wouldn't use, unless your amp has a 2 Ohms setting (I think I've seen this with some Mesa Boogies).
As a final note, it is important to realize that a speaker doesn't have a constant impedance anyway. If it is rated at 8 Ohms, then you will find frequencies at which its impedance is a bit smaller, and you'll find a wide range of frequencies where the impedance is considerably higher. That knowledge could be a motivation for not getting too neurotic about a small (!) mismatch.