aftermarket speaker design is a little different with original, except hte wattage, the oem speaker design to have more mid bass range of sound output than aftermarket speaker which are linear. the oem design like to to compensate the oem door that were not sound dampened, to sound like it has some bass.. you will find out the oem speaker bass are more like echo rather than bass.

just fyi, i had tried many way to make a myvi rear spk sounds good but i gave up after all. the location is actually not ideal for front, and even for rear passenger.

if you wan your overall system sounds good, the 4" spk remain, and add in a subwoofer which can do slight higher range of sub to mid bass like 60-120hz to compensate the size of your 4inch spk. It sounds a bit weird to use a sub covering a mid bass driver (6.5") but if u tune it right, the simple setup will last u at least 1 to 2 years before u think of an upgrade.