If your still concerned about getting everything to match up there are a few things you can do. By wiring 2 4 ohm speakers in series you will get 8 ohms by the time the signal gets back to the amp. If your not sure how to wire in series, let me see if I can explain... Take the positive OUTPUT from the amp and connect it to the positive INPUT of the 4 ohm speaker. Then, take the negative OUTPUT from that 4 ohm speaker and connect it to the positive INPUT of a second 4 ohm speaker. Lastly, take the negative OUTPUT from speaker #2 and connect it to the negative INPUT of the amp. Now you will have 8 ohms at the amp. Basically, all you do when wiring anything in series is add up all the resistance (r1+r2+r3...= total resistance) to get what you need. BUT, if your using any sort of passive filter (capacitor or coil) as a high pass or low pass filter things will change. In this case a multi-meter will be your best friend. Please feel free to IM me if you need more help. There have been books written on this subject, and I don't want to write my own here lol. Let me leave you with the 2 formulas you'll need to figure whatever you need for your speaker wiring...

Series...R1+R2+R3...(etc.) = total resistance.

Parallel...R1+R2+R3...(etc) divided by the total number of componants = total resistance.

Example: 2 4 ohm speakers wired in series will give you a total resistance of 8 ohms.