hi neo,
i am looking out currently for exactly the same thing for my new alto as you are for ur spark - 4" drivers. dunno abt whether u are going to install rear speakers in the already limited bootspace of the small hatch.
i am at this point of time, not considering putting up big 6x9 speakers in the rear. so i am looking out for such 4" drivers which will give good, clear sound across all ranges - highs, mids and good bass. even if they may not fill the car sound, the front seats should get punchy sound. now i am writing all this so that knowledgable others can help me / us.
the little research that i have done for high-end 4" drivers has shown me that there are component speakers available in which there is a 4" bass driver, seperate tweeter and a crossover unit. these component units are available in 6" and 5" also. have not been able to find demo in 4", but the 6" ones which i auditioned sounded great.
there are 2 brands which i know till now - kicker and dls. auditioned both (6") and found kicker with better punchy bass, but dls with excellent clarity and soundstage. these brands also figure in top high-end car audio brands in various forums across the net.
you may check these brands. if you come to know of any other brands, pl do let me know....
cheers,
vilfy