A 4 ohm speaker will be louder than an 8 ohm speaker. Unless you're talking megawatts, or a really wimpy speaker, just start with lower volume and you probably won't hurt anything.
There are really just two very simple things to look at. One is the output of the amp. Typically, you will see that the amp is rated to put out some number of watts (say 300) into 8 ohms, and much more, close to double (say 500) into 4 ohms.
The other is the power handling of the cabinet. Often there are two numbers, continuous and peak, so a given cabinet might handle 200 watts continuous(ly), and maybe 400 for peak (brief) periods.
It's actually considered safer to have more amp power, up to the peak rating of the cabinet or even higher, as long as you're not constantly playing at 11. So if you have the specs available, see how much power your 4 ohm cabinet is rated to handle, and then compare that to what your amp puts out into 4 ohms. Or if you don't know what your amp does into 4 ohms, take the 8 ohm rating and double it. As long as that number isn't grossly more than the peak speaker rating (even twice as much is generally fine), you shouldn't have any problems.
Things get trickier when you get down to 2 ohm loads, since some amps don't like to go there. But most contemporary amps will handle 8 and 4 ohm loads fine, just putting out more power into less resistance. And remember they're only putting out their rated power when you have them cranked, and you play something loud or percussive, most of the time it will be much less.
-Bob (no electronics whiz either)
