Not sure if this is a superstition - do we always have to shift the gears from 5-4-3-2-1 style or can I shift the gears directly, say to 2nd gear from 5th or from 4th to 2nd. (of course, only when there is a sudden drop in speed)? I have been instructed by many to follow the 5-4-3-2-1 way... Please advise.
it is always advisable to go to use gears to slow down under heavy braking,I usually go from 5th to 3rd directly & introduce half clutch & gently introduce full when slowing down from a high speed
it is bad if lets say on your alto 2nd gear redlines around 80km/hr & your travelling at 120 in 5th & you suddenly brake hard & change to 2nd in an instant,your engine will shoot from 2100 rpm in 5th gear to red line,
so you can go to 3rd gear instead(may vary if car is geared short/tall & speed) when slowing down from high speed but its not at all necessary to follow 54321 downshift.
As for your difference between 4 cylinder & 3 cylinder question between santro(1086cc) & k10(998cc) its not a big deal,as small cc petrols run smooth,
but 4 cylinder or 6 cylinder cars will be have better engine balance when running compared to a 3 or 5 cylinder car, difference in vibrations(noise also) in diesels can be huge depending on cylinder configurations..
Maruti just found it cost effective & viable to take the 4cylinder(1197cc) k-series engine & make it smaller by cutting out a cylinder & fitting it to A-star & alto k10 ,like chevy did in diesel beat with fiat 1.3 multijet engine,as you can see in the beat the 3 cylinder config makes it run nosier!