I just bought an '02 Honda Civic SI hatchback that

You have to understand impedance to do this. how many ohm are your 12,s`? you can run all 4 subs in mono off one amp if wired right. it works like this, lets say all 4 of your speakers are 4 ohms. if you wire 2 4 ohm speakers in a series it looks like one 8 ohm speaker to the amp. if you wire them in parallel the amp sees it as 2 ohms, ps most amps cant handle less than 2 ohms in sereo 4 in mono bridge. so no if you take one pair say the tens run them in series now take the 12 run them in series. now take and parallel both sets to amp and you have a 4 ohm load. to better understand do a google search for series parallel circuits. also you amp should be marked for bridging if not look up the amp on the manufacturers web site most will let you download the owners manual for free if in doubt about your amps ability keep it at or above 4 ohms