u can get foam baffles that r form fitted to mount under ur 4x6'*.. however they wont do much good, since interior speakers r for reproducing mid and high frequencies and using a baffle will only help a little with low frequencies, unless its a 2 way speaker, then ur mids will improve a touch since the woofer will take care of the mids.
u can run ur front and rear stage with one amp. its all in the settings. even if u dont have seperate gains for front and rear (if its even a 4 channel amp) just make sure ur not sending much bass at all to ur interior speakers. since all the speakers will be recieve the same amount of power, the ones with lower rms power handling will be pushed harder, and bass is what will destroy ur speakers.
and as previously mentioned, watch ur impedance. if ur using a 2 channel amp and are running front and rear Left of the left channel and front and rear right off the right channel, make sure the final load to each channel is at the lowest impedance that amp can handle per channel or higher, not below.
Most 2 channel amps r 2 ohm stable per channel or 4 ohms stable bridged, but it varies.
ur ur running a 4 channel amp and each speaker is getting its own channel, u should have no worries
1992 Bonnie SE, freshly overhauled, 6000k HID'*, SensaTrac Struts, 32mm Tubular SSEI sway bar
Kenwood HU , JL 1000/1, 8W7'*, 15" CVX'*, 18" DC lvl4