Multiple forms plus opacity help.

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C#3.0

C#

C#4.0

Hello, I have 4 forms. Forms 2-4 are just storing the preferences for form 1. What do I need to do so that form 4 will make changes in form 1?
Here is an example:

On form 4 I have a track bar named "trackBar1"

now, depending on the track bar's value, I want form 1 to be set to a certain opacity. Here is the math.

.Opacity = (trackBar1.Value * 0.01);

But now, i just need to send this between forms. I have done a lot of google searching, and this is my last resort. thanks.

EDIT:

I forgot to mention, is there a property, in which I can make a window not be interact-able - if that's a word.. I just want a certain form to not accept any mouse clicks, i want the form behind it to accept the mouse clicks.