Well, Clinton sort of answered himself while I was compos(t)ing; but my babblings may be useful anyhow. The 4 ohm speakers can be used if you can find the right "cheap accessory" (see following).

For a normal "home system," an actual "acoustic power" of 7 to 10 watts will be as much as you want. Anything more than that in small enclosed spaces risks damage to your hearing. Really cheap speakers can be quite efficient, so quite a bit of cheap commercial/consumer stuff actually has 7.5 or 10 watt speakers built in. As you progress into "better" quality speakers, the speakers become less efficient, so you need more "electrical watts" to get that "10 acoustic watts."

Running an amp that has significantly higher power rating than the speakers is quite common, works very well in normal circumstances, and shouldn't cause you any problems. I ran a 40 W/channel amp into a pair of 7.5 W speakers salvaged from an old phonograph for several years - until my son, having bought a new bass guitar and NOT buying an amp, snuck into my place to try it out. Full gain - one WHANG - cones shattered, wires melted. (No damage to the amp.) "But dad, I never turned it up above 3!" ... "%$@#!!"

The 40 W speakers should work fine with a 50 W amp, but the 4 ohm vs 8 ohm mismatch is a little more troublesome.

Most equipment capable of 50W per channel output would normally allow you to select 4 or 8 ohm speaker outputs. There's no guarantee that yours does, but you need to look and be sure. Sometimes this feature appears as a different set of screw terminals for the alternate speaker impedance, and sometimes as a switch that lets you make the appropriate connection to a single set of jacks. (Clinton answered this?))

If your amp does not allow you to select for 4 ohm speakers, the simple expedient is to put two of the 4 ohm speakers in series on each channel. You MUST make sure that the speakers are "in-phase" if you do this, but that's a trivial thing.

Assuming that you don't want 4 speakers instead of 2, you really should check the local Radio Shack and see if you can find an "L-Pad" to match the speaker impedance to the amp output impedance. Essentially, this is just a pair of (sometimes 3) "resistors" that looks like 8 ohms on the amp side and 4 ohms on the speaker side, all nicely packaged for you. The pad should have a power rating at least as high as the speakers, and you may want to match the amp power rating here if it's easy. The last ones I bought were when vacuum tubes were the rage, but they weren't more than a couple of bucks then. They may be a "catalogue" or special order item now, as they're not as much in demand.

Note that the "couple of resistors" does oversimplify things. The amp anticipates an "inductive load" and just using resistors from your junk box seldome works very well. The commercial pads will give much better results.

If your amp happens to have a vacuum tube output stage, the vacuum tube amplifier is a "voltage gain" device, and produces the voltage needed to produce the desired power in the intended (8 ohm) load device. If you put a 4 ohm load on it, you'll get up to twice the current - or 4 times the power into the speaker. At low power, relative to component ratings, this shouldn't cause real damage, but is "stressful" on components both in the amp and on the speaker. It can also make volume control "touchy."

With the more common transistor output, the transistor amplifier stage is a "current gain" device, and will try to deliver the current needed to produce the desired power in the expected (8 ohm) load. If your load is only 4 ohms, that current will only produce half the expected voltage, and you'll get half the intended power into the speaker. This will force you to set your volume control up substantially to get desired audio volumes - which again stresses components in ways not intended in the design.

Neither of the two kinds of amplifiers is a "pure" current/voltage gain device, so how the actual mismatch effects come out is just a "general tendency" description here. Any vacuum tube output will have a transformer that acts as a significant "ballast" and can change things significantly. A transistor output stage must have a few resistive output stage components to protect the circuit in the event you turn the amp on without a speaker connected, and these may significantly affect the actual "deviant behaviour" you'll see. Either of these design "features" should tend to compensate for minor mismatch between amp and speaker resistances, so you might get by with 4 ohm speakers on 8 ohm outputs at low power levels - but a real fix is strongly recommended. You're not likely to be very satisfied with the sound you get, even if it doesn't damage anything.

John