<font color='#000000'>If you want 4 ohm impedance, then you need either a single 4 ohm woofer or two 8 ohm woofers in parallel. Since the woofer consumes about 80-90% of the output from the amp, it determines the overall impedance of the system.
Starting out with the enclosure is designing a speaker bass ackwards. The usual sequence of events is select the drivers, based on compatible frequency responses, sensitivity and power handling. Next, design an enclosure for the woofer and, if it's ported, tune the enclosure to the woofer. Next, frequency response and impedance measurements are taken from the drivers mounted in the enclosure. Some adjustments like tweeter position on the enclosure baffle may need to be made to ensure the smoothest frequency response possible. This makes designing the crossover easier. After the response measurements are finalized, the figures are fed into a computer optimizer program which begins the crossover design. Several iterations of a crossover may be built and tested before a final configuration is decided upon.
As you can see, it's highly unlikely you'll be able to find an off-the-shelf crossover and a cabinet that will be suitable for a given set of drivers. Since this appears to be your first foray into building a loudspeaker, it might be a better idea to build a kit where a competent designer has done all the heavy lifting for you. Check out the offerings from Parts Express (the BR-1, Titanic subwoofer), Dennis Murphy at Murphyblaster Productions, Rick Craig at Selah Audio, Northcreek Music, or GR Research. All of these guys are first-rate speaker designers and each has several designs to fill just about any need you may have.</font>