clarification about amdahl's law
No, you're way off base, and Amdahl's Law does not apply in this case.

It's a simple rate problem, like you had in your first algebra class. You have four subsystems, each of which contributes the same amount of time to the overall operation:

T1 = T2 = T3 = T4

Ttotal = T1 + T2 + T3 + T4

Ttotal = 4 &times; T1

If you want to divide the total time in half while keeping the subsystem times equal to each other, just divide both side of the last equation by 2:

Ttotal/2 = 4 &times; T1/2

In other words, you have to speed up each subsystem by a factor of 2; i.e., each one must run in half of its original time.

The second problem is a little trickier. Let's call x the amount of time by which the second two subsystems are sped up, which means that the first two subsystems are sped up by 2x. Now we can write:

Ttotal/2 = (T1-2x) + (T2-2x) + (T3-x) + (T4-x)

Solve for x:

6x = T1 + T2 + T3 + T4 - Ttotal/2

Making the other substitutions we know about:

6x = 4 &times; T1 - (4 &times; T1)/2

6x = 2 &times; T1

x = T1/3

Therefore, the first two subsystems must run in 1/3 their original time (T1 - 2x), and the second two must run in 2/3 of their original time (T3 - x).