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....