"TVC15" - Sean what a great old rockin' Bowie tune; should make for some good breakin' jams!
Now for those dummy loads that Tvc wants to build... I use 20 watt Ohmite wirewounds in a series-parallel arrangement to achieve an 8 ohm measurement-load in my shop, but for this purpose I'd want a lower resistance network. 4 ohms is a nice number but that value may not be available in high power at RadioShaft? I don't know what resistance values they have available & in stock without looking it up, & I'm not about to attempt teaching Ohm's law here on a forum.
Tvc try to get a quantity 16ea. (16 for each channel & you have four total channels) of 5 ohm power resistors, preferrably wirewounds or ceramics rated 5 or 10 watts apiece, although 5 watts would be acceptable. Just get whatever you can find easily as it is not all that critical. Solder up four strings ea. of 5 ohms R's in series. Now solder those 4 strings parallel to each other, to achieve a 5 ohm load network. The leads in between the series R's strings must not touch each other, only the ends of the strings can connect together. Attach some 18 gauge stranded copper wire pigtails, then crimp or solder some spade lugs that will fit your amp's binding posts. While fabricating, you could use some stripped THHN insulation or tubing etc. as spaghetti for insulation on the network's leads to prevent short circuits. It must withstand a LOT of heat, so if you build carefully then insulation may not be needed & in fact could melt & burn, so watch out! If you use 5 watts R's then you'll have an 80 watt load, or if using 10 watt R's then it's a 160 watt load which is plenty for this job.
Now as you apply the (test signal) or music signal into the amp, it will of course begin to output power relative to the applied input level. DO NOT get carried away such that you go into clipping. If you don't have an oscilloscope, be very careful as input drive is gradually increased. The resistor dummy load will begin to warm up; it can be warm, in fact it will get hot & that is OK. But too much power will burn up the amp or the network or both, so use caution & err on the low side especially if you don't have a meter or scope to monitor your output power levels. Also: place the dummy loads on a glass dish or ash tray so you don't burn up your workbench.
Sorry for the length of this post I'm trying to be reasonably specific without going on & on...