Goop you have a lot to learn about turbos in general.

First off, twins are not better than single. Single turbo's will always have more top end than twins, which will spool better low and maybe mid, but will then give out. The specifics are much more in depth, but know that the end result is such.

Second, only SOME 4cyl's can run 15psi. First off you have to consider the size of the turbo. A T-25 will not flow near as much air at 15psi as a T3 or 4 series (not to mention hybrid turbos and bigger). So to say 15psi doesn't give the whole story. Next, the ones that are running 15psi (and much more) are either a) motors that are build to be turbo driven from the factory or b)rebuilt aftermarket to handle boost.

Twin turbo is easier on a V-designed motor because of what Donnie stated, you have manifolds on seperate sides of the motor. If you go single, you have to create much more piping to merge the two flows of exhaust together. But remember that in the end a single turbo will make more power.

Trust me, a single turbo kit with a t-44 turbo at around 8psi will make you loose traction on street tires in 3rd gear. 8 psi does not sound like a lot, but it can be...all depending on the size of the wheels and housings of a turbo.