I have created a simple plug-in that replicates what you are trying to do (link is at the end of this reply). I use forwarding functions to set the parameters but it looks like you want to access each sine component individually. I put both ways of doing this so check the code carefully.

// in MultiSine.h
// array of four
CSine sineArray[4];

// in the plug_in.h
CMultiSine multiSineArray[2];

// in userInterfaceChange() as an example:
multiSineArray[0].sineArray[0].setFo(m_fOsc1Fo);
multiSineArray[0].sineArray[1].setFo(2*m_fOsc1Fo);
multiSineArray[0].sineArray[2].setFo(3*m_fOsc1Fo);
multiSineArray[0].sineArray[3].setFo(4*m_fOsc1Fo);

As for your question on indexing variables; you can do this indirectly by using the RackAFX linked list of GUI control objects. Each GUI control (slider, button) on the UI is indexed from 0->39 for the sliders and 41-44 for the button banks). This ID value is also shown at the top of the table when you setup a slider.

Each control is encapsulated in a CUICtrl object. These are then stored in a linked list in the plugin object. The CUICtrl object has a pointer to your member variable and uses this to change the value as the user moves the slider. You can access this pointer and de-reference it to use it. For example in userInterfaceChange again, and using my method of calling one function that internally sets the 4 frequencies:

// accessing the m_fOsc2Fo with an index from the UIControl array
// get the pointer; I know it is slider #1
CUICtrl* pUICtrl = getUICtrlByControlID(1);

// check to make sure its valid before using, just to make sure...
if(pUICtrl)
// use it by de-referencing the Cooked data
multiSineArray[1].setFo(*pUICtrl->m_pUserCookedFloatData);

You can get the whole plug-in project here:

http://www.willpirkle.com/Downloads/ArrayOfObjects.zip

Will