dynamics alignment

[Werner]

Hello,

I write four-voices in a system of two staves with two voices in each stave.
With ^\f or _\f e.g. I can get dynamics over the upper voice or under the 
second.
I also found out, that I can align them with e.g.

\override DynamicLineSpanner #'staff-padding = #4.0

Unfortunately I don't understand, what means

\override DynamicLineSpanner #'Y-extent = #'(-1.5 . 1.5)

That is my first question.

Second:

I would like to have dynamics for the single voices (I use ^ for voiceOne 
and _ for voiceTwo) but also dynamics for all 4 voices. These should be 
aligned just in the middle between the two staves.

Do I have 
to write a fifth voice containing only s s s ... for that 
or
to override DynamicLineSpanner again and again in one of my four voices
or 
is there in lilypond a smarter possibility?

Greetings

Werner