The only way to present a 4-ohm load to the amp with those speakers is to run them in parallel without a crossover, in which case your 15 will be getting low bass signals at full power.
Oversimplifying here a bit, a passive crossover prevents the signal from going to a particular driver by rapidly raising the impedance of the signal on the "stop-band" side of the crossover frequency, thus "impeding" the signal as it tries to get through. For example you would use a large capacitor in series with the 15 inch woofer to roll off its response below 200 Hz, and as we go down in frequency below that point, the capacitor's impedance rises so it progressively resists passing the signal more and more. At 100 Hz the capacitor might add 25 ohms, and at 50 Hz more like 40 ohms. So at 50 Hz the impedance into that woofer + capacitor might be 48 ohms, while the impedance into the 18" woofer will be 8 ohms (oversimplifying for the sake of clarity). Doing some quick arithmetic, in our example here at 50 Hz, roughly 85% of the signal will pass through the 18" woofer, and 15% through the 15" woofer, so the latter will be adequately protected. The resulting load that the amplifier sees will be, for all practical purposes, 8 ohms.
At the same time, the response of the 18" woofer is probably being rolled off on the top end by an inductor in series with it, so you have the same thing going on as we go up in frequency as well. The result is a roughly 8 ohm load throughout the crossover region.
The real world has more complex interactions, and often steeper, more complex crossovers are used, but the basic principle illustrated here remains the same.
The "frugal" solution is to run the sub fullrange, protect the 15 + horn with a large series capacitor, and accept that your amp isn't going to see a 4-ohm load in the bass region. I'd suggest a fairly low crossover frequency to allow your 15 to share more of the low bass load, like maybe shoot for a calculated ballpark 125 Hz crossover frequency. Like, maybe 150 microfarads of capacitance in series with the 15 + horn. I'd use Dayton 240 volt polypropylene caps from Parts Express, and three 50 uF caps will run you about $35 plus shipping. I do not recommend electrolytics because they tend to sound a bit harsh. If this doesn't adequately protect your 15, cut back to 100 uF. If your 15 seems unstressed and you'd like to squeeze a bit more low bass out of it, bump up to 200 uF.
Hope this helps.
