You
only use 4 cyl springs if you have a
quarter mile *only* car They are used to help with the weight transfer to the rear during take-off.
As always, I have to ask
why do you want to lower your car?
o Handling?
o A little better handling and a slightly more aggressive a lower look?
o
Looks *only*?
o Quarter Mile *only* car?
o and so on.
I help people that are *honestly* looking for better handling. For the first two cases, as mentioned, the Bullitt/Mach1 Springs are often your best bet, especially if you're on a budget.
If you want the (IMHO, ricer) lower look, then you can do it cheaply, as many people have done. Or, you can spend a lot more money. Regardless, either way, you would still have a car that in
real-life handles
worse and has
many suspension geometry problems compared to the cheapest base stock Mustang. But, if you're interested in *only* looks (often slammed), then handling is not the concern.
As I said, it all depends on
why you want to lower your car.
For a qtr mile car, yes, people go with the 4 cyl springs to get better weight transfer on take off. But, otherwise, it's honestly a very foolish mod for a mainly street car.
And, yes, I put
my money where my mouth is.

When I re-did my suspension, I did it from an engineering point of view.
http://www.veryuseful.com/mustang/tech/misc/Mustang1986_with_Mach1_springs/